Previous unserved US destinations for British Airways are Anchorage, Detroit, Ft Lauderdale and Oakland. British Caledonian also briefly served St Louis in 1981/82.
I think you meant "served"? FLL and OAK were recently served from Gatport Airwick and ANC was the stop on the BA5/6 on the way to Tokyo ending only with the delivery of the B747-400 and Russian airspace being opened up. DTW was served with a Houston tag on til 2006 as IAH was a LGW direct due Bermuda 2.