British arms may be in the headlines, but British arts have been making gentler conquests for centuries. It is well to be reminded of them in the announcements of the largest festival of British arts to be held outside Britain. It is ''Britain Salutes New York,'' taking events in theater, dance, film, music, painting, sculpture to more than 50 locations in New York in the spring and summer of 1983. The occasion?It couldn't be more appropriate. A peace treaty - or rather the bicentennial thereof - ending the Revolutionary War in 1783.