[Peko]

Legally the position is just the same as in pre-Covid times, it is the responsibility of the passenger to have the right of entry to the destination country (whether by nationality, residency or visa / ESTA).

If a flight operates, the airline is not legally liable for cancellation due to this condition not being met, it is treated just like a decision not to travel. The fact that the borders have been closed to many foreign nationals, or that the FCO is advising against travel, doesn’t change the legal position. Theoretically the airlines don’t even need to offer vouchers, but most have taken the decision to do that.

OP - you don’t mention when and where your flight is, but for as long as BA is offering vouchers, you are probably better to sit tight and wait to see if the flight cancels, unless you have watertight insurance that will pay out if you cancel now.