Heavyweight lobbying campaigns are nothing new to Washington, but this one has a twist: It is being mounted by a venerable 300-year-old British institution and backed by Her Majesty's government.

Top executives of Lloyd's of London, the best-known insurance organization in the world, have jetted across the Atlantic to urge Congress and the Treasury Department to protect favorable tax treatment for thousands of Lloyd's U.S. investors.

Lloyd's, known for insuring everything from quarterbacks' arms to nuclear reactors, also has hired an American law firm to help lobby against a little-noticed provision of a House-passed tax bill. The provision, included in an $11.2 billion tax increase bill passed by the House last month, would reduce the financial incentive for Americans to become Lloyd's underwriters -- suppliers of funds. In recent years, more and more wealthy U.S. investors have joined Lloyd's hundreds of syndicates.

The endangered U.S. investors, or underwriters, also are calling their elected representatives to protest, at Lloyd's instigation. And the British government is chipping in: The ambassador here, Sir Antony Acland, has written the chairmen of both congressional tax committees in opposition to the provision, and embassy officials have met with representatives of the U.S. Treasury Department.

The British Treasury Department is said to be pressuring the Reagan administration as well -- all to preserve what some members of Congress see as an unfair tax advantage.

"The Lloyd's investment is a way to postpone paying taxes and earn income on your income for three years. That is not in the spirit of 1986 tax reform," said Rep. Fortney H. (Pete) Stark (D-Calif.). "It is a special advantage in an industry that pays very little taxes to begin with."

Although Lloyd's executives don't come out and say that they would move their money out of the United States if their lobbying campaign fails, the threat seems implied. Lloyd's representatives pointedly note that it has $8 billion invested in the United States, 70 percent of it in government securities.

"About 60 percent of our business is U.S.-based," said Alan Lord, Lloyd's deputy chairman and chief executive, during an interview in his suite at the Willard Hotel. "If we were faced with taxation under this system, the least we would have is very considerable uncertainty. We want that out."

The provision is not included in the Senate version of the tax legislation, which is expected to be considered shortly. The Treasury has not taken an official position on it, but officials say it is not clear that Lloyd's investors have an unfair advantage. They also note that the bill's provision may violate a longstanding agreement between Lloyd's and the Internal Revenue Service.

It is perhaps fitting that Lloyd's, which still retains the cachet of a gentlemen's club, negotiates directly with the IRS. Practically since its origins in Edward Lloyd's London coffeehouse in the 17th century, it has occupied a special place in the United Kingdom. "They see it as the last vestige of the British Empire," said a lobbyist who opposes Lloyd's position.

Lloyd's is not a company, but instead functions more like a stock exchange. It is made up of 32,000 individuals who supply the funds that, with the premiums paid by buyers of insurance, make up the insurance pools that pay claims.

About 10 percent of those individuals are American, and the proportion rises each year -- as does the size of Lloyd's, which trade publications say will take in an estimated 3.3 billion pounds (about $5.8 billion) in premium income this year. According to Lloyd's officials, the syndicates will write 10 billion pounds' worth of new policies in 1987.

Anyone who wants to put his money into Lloyd's insurance faces some strict tests. Potential investors must demonstrate that their wealth exceeds 100,000 pounds ($177,000), not including the value of their homes. Then they have to pledge at least half that amount to the Lloyd's insurance pools -- if they pass muster during the mandatory personal interview in London, travel expenses paid by the applicant.

The potential return is enormous if the member -- called a "name" -- invests in a Lloyd's syndicate that gets few claims. At the end of the accounting period, the investor is entitled to his or her share of the leftover premium income, plus the investment income on his deposit, and the deposit itself.

But the potential risk makes Wall Street look like a sure thing. Liability in a Lloyd's investment is limitless. If the syndicate must pay out more in claims than it took in, the names must make up the difference -- even at the cost of their personal fortunes.

Forbes magazine reported last year that a $131,000 deposit in one particular Lloyd's syndicate insuring marine risks would have yielded a profit of $157,462 in 1983 (Lloyd's keeps its books on a three-year delayed basis to allow claims to settle). However, the same deposit in a casualty insurance pool would have cost the investor an additional $421,176 to cover losses.

The three-year accounting system grants special tax advantages to the Lloyd's U.S. member, courtesy of the agreement between Lloyd's and the IRS. Until the syndicate's accounting year is closed, more than three years after it begins, investors do not have to pay taxes on their profits. Nor do they receive the profits, but their money can build up faster if it is untaxed.

Lloyd's American names also pay taxes as individuals rather than at the corporate rate, giving them a top tax rate of 28 percent next year rather than 34 percent. Individuals in similar U.S. insurance organizations pay tax on their profits as a corporate entity, and what remains is treated as a dividend and taxed again at individual rates.

There is considerable disagreement, even within the administration, about whether the Lloyd's scheme is fair. But there is less debate over the contention that the three-year deferral of taxes allows Lloyd's to attract more capital, and thus grow more quickly.

"The bottom line is, what you are trying to do is expand your capacity to write insurance, and you can only do that if you have capital," said John D. Raffaelli, a lobbyist for a Dallas-based insurance group, organized similarly to Lloyd's of London, that wants individual tax rates for its members.

The trade association for the reinsurance industry, which sells policies to retail insurance companies to cover their risk, also is lobbying to change current treatment of Lloyd's investors.

Those investors, it would appear, are fairly well-off. Aside from the high wealth requirements, Lord, the Lloyd's deputy chairman, estimates that American "names" are richer than their British counterparts simply because the United States is "a richer country."

Insurance industry magazines also estimate that U.S. names account for a larger proportion of Lloyd's policies than their number would indicate. National Underwriter magazine said earlier this year that the 3,200 or so U.S. names will write $4.1 billion of the $18 billion in policies Lloyd's is expected to produce this year.

"People I know, casual acquaintances and others, have become Lloyd's names. They are the kind of people who could not spell insurance the same way twice, and have a lot of money," said Rep. Stark.

It is not clear how much tax revenue would be raised by taxing Lloyd's syndicates as companies, which the House bill would do. Lloyd's officials say the syndicates would be reorganized overseas, so that no income would be subject to U.S. tax. But insurance sources point out that billions of dollars in premium income accrue to Lloyd's U.S. partners each year, raising the possibility that tax collections in the hundreds of millions could be at stake.

The U.S. names seem to feel their pocketbooks would be affected if the provision became law. Some have been calling House members to protest the provision -- in no uncertain terms.

Lloyd's also has hired the firm of Davis & Harman to lobby against the provision, using, among others, attorney Gail B. Wilkins. She is married to William J. Wilkins, staff director of the Senate Finance Committee. Wilkins said he had not discussed the Lloyd's issue with his wife or any member of her firm, but had met with John C. Richardson, an attorney with Lloyd's New York firm of LeBoeuf, Lamb, Leiby & MacRae.

Lloyd's executives said that in their discussions with members of Congress and staff aides about the provision, they had not in any way threatened to move the $8 billion-plus fund, which is invested here so it can back up U.S. insurance claims. They said they only pointed out that payment of U.S. taxes on the income from the fund amounts to some $170 million per year.

"We would have to consider a whole range of matters" if the provision became law, Lord said. "Our only strategy is to make people understand the structure of Lloyd's, and that that structure makes the bill inapplicable -- it's like trying to feed an elephant on canary seed."