Using a historical occasion to make a political point, President Reagan declared yesterday, "The enemy at home is no longer the Red Coats but red ink," as he signed an official proclamation marking the 200th anniversary of the British surrender at Yorktown.

Reagan spoke at a White House ceremony where he designated Oct. 19 for the observance of Lord Cornwallis' surrender to George Washington, which brought the Revolutionary War to a close.

Reagan will attend the ceremony, which is being held just three weeks before Virginia's hotly contested election for governor. French President Francois Mitterrand also will attend to commemorate the French contribution to the British defeat.

The British, in a relaxation of their earlier position, will be represented by their ambassador to Washington, Sir Nicholas Henderson, and "one or more distinguished visitors from London," though not the queen or prime minister, said Charles Anson, a press attache at the British Embassy. "We were glad to be invited," Anson said. "The spirit in which the Americans are celebrating is more forward-looking than it was in the past . . . You have to take a wide view of these things. We're all friends now."

In his remarks yesterday, Reagan said, "We Americans today are not often asked to make such sacrifices" as the soldiers at Yorktown. "Nevertheless, it will take new determination and new resolve to preserve the treasures of our revolution."

Reagan said the critics of his tax-cut and budget-cut program which takes effect Oct. 1, have been "so quick to carp and complain, so ready to retreat even before the program has begun." The president said he had two questions for them: "If not us, then who? If not now, then when?"