Revenue ticket is not a problem since I have done that before. But for redemption, particularly for 2-4-1, BA only allows a UK to UK. Am not sure what the basis for this is since I did try to do a similar thing few years ago and was told that I can’t do it but I can’t find any T&Cs for it. I suppose it is to prevent people potentially abusing it like LHR-LAX-LHR-JFK-LHR with a single voucher rather than 2 vouchers!!!