For some Britons, it forms the felicitous barrier that has kept the country pure -- Shakespeare's "moat defensive . . . against the envy of less happier lands." A geological remnant of the late Ice Age, the English Channel has defined much of the character of this island, seagoing nation.

For many others, however, the channel is an inconvenient ditch. Expensive and time-consuming to cross by ship, it separates Britain and its economy from true integration with Europe.

Today marked the deadline for submission of industrial proposals to the governments of France and Britain to span that ditch. It brought the official unveiling of what local wags have dubbed the "brunnel," the "chunnel," the "brubble" and a handful of lesser multibillion-dollar variations of bridge and tunnel, road and railway projects that all promise to revolutionize the channel crossing by the mid-1990s.

Plans to build a tunnel under the channel or a bridge spanning it for two centuries have fallen victims to skeptical engineers, exorbitant costs or governmental trepidation. The shoreline near Dover's white cliffs, the closest point to the French coast 22 miles away at Calais, is dotted with the industrial debris and abandoned boreholes of past efforts.

Despite the failures, Britain and France still think it is worth trying to build a road -- above or below the water -- between them. Following a go-ahead last year from Prime Minister Margaret Thatcher and President Francois Mitterrand, Anglo-French consortiums have been drawing up projects that meet the principal criterion of their governments -- that no public money will be needed.

Each of the main contenders promises to be able to raise all the money needed from private sources in Britain, France and on the international financial markets, with returns of up to 20 percent to investors. Each promises tens of thousands of construction jobs and a revitalization of heavy industry in both countries. Each promises that it will accommodate a projected doubling of cross-channel traffic by the turn of the century and will constitute what Thatcher last year called "something our generation can perhaps do for future generations."

Each of the proposals is roundly opposed by a separate group of predictable critics. They range from the operators and employes of the cross-channel ferries, which last year carried about 40 million passengers and three-quarters of a million freight trucks between Dover and France, to environmental groups fearing destruction of the pastoral countryside. Most local governing bodies in the seaside area fear a net loss of jobs and income once the building is over and the ferries disappear.

For the next three months, the two governments will listen to the critics and study the proposals. They have promised to decide on one, or none, of the projects by late January.

So far, the front-runner appears to be the "brunnel," the bridge-tunnel combination proposed by Euroroute Ltd. It is the most expensive and futuristic, and has the advantage, in the words of its voluminous and polished press kit, of "freedom of choice" for channel-crossers.

On Euroroute, a vehicle is driven from Dover five miles out into the channel to an artificial island, where a spiral ramp descends to a 13-mile tunnel. The tunnel ends at another ramp, island and bridge five miles off the coast of France. A separate, parallel rail tunnel also runs 23 miles, coast to coast.

The projected cost for Euroroute, without loan interest or cost overruns, is $7 billion, to be paid by shareholders, institutional investors and bond flotations. Sir Nigel Broakes, chairman of Euroroute, heads Trafalgar House, a British engineering, construction and shipping firm with projects worldwide. Euroroute's advisers and backers include Barclays Bank, British Steel and Kleinwort Benson, and its French partners are Banque Paribas and Societe Generale.

Surveys done by Euroroute indicate that most channel-crossers prefer to drive their own vehicles, an advantage of the "brunnel" that also is said to appeal to Thatcher.

But many here are banking on the "chunnel" project, proposed by the Channel Tunnel Group headed by Sir Nicholas Henderson, former ambassador to the United States and to France. For $4 billion, it offers a modern variation of a plan proposed to Napoleon Bonaparte in 1802 for a double borehole dozens of feet under the chalky channel bed. It also closely resembles an Anglo-French project begun in the last decade and abandoned by the British government in 1975 with only a half-mile hole near Dover to show for the effort.

The "chunnel" is rail only, with two parallel tunnels, connected by a separate service tunnel, for electric trains on which cars and trucks would be loaded for high-speed shuttle. Its designers promise a train leaving every five minutes, and a capability of 4,300 vehicles per hour.

Henderson's backers include NatWest and Midland Banks in Britain and Credit Lyonnais and Banque Nacionale de Paris.

The "brubble," or Eurobridge, project envisages separate road and rail crossings costing about $7 billion. Vehicles would travel on a road inside a multidecked plastic tube suspended from eight towers built across the channel between Dover and Calais. Trains would use a rail tunnel bored beneath the sea. Critics say a bridge spanning the busy channel sea lane is too dangerous.

A fourth proposal, Channel Expressway by Sealink British Ferries, one of the channel ferry companies, is for a $3 billion rail-and-road tunnel.

There are also some dark-horse contenders. Eurolink is a plan by four London businessmen for a road and rail bridge, with the bridge piers incorporating hydroelectric generators that would produce power for sale to Britain and France and allow bridge users to travel free.

A businessman from Bournemouth, on the southwestern English coast, submitted a plan to the Department of Transport today but did not make public its details. A retired French inventor turned in a proposal that, like the Euroroute "brunnel," involves a combination of bridges, tunnels and artificial islands.

Flexilink, a consortium of ferry and port operators opposed to any project that takes business away from them, released a proposal last night for a fleet of ferries twice as big as the current ones, charging half the fare of the current car-ferry system.

Tonight, British Transport Secretary Nicholas Ridley described "a complex process of evaluation" that the two governments will begin Friday. "They will look at the environmental impact, the economic effect on the region, landscaping, everything."

The governments will look at legislation needed for any of the projects to go forward, and projected alterations in treaties. "Everyone affected by the proposals will have a right to make representations," Ridley said. "We do want everybody to be consulted."

But, he warned, "we do not want to take 20 years to do it."