Do keep in mind that non-US carriers charge fuel surcharge (YQ), so if you are someone who redeems awards in economy, its almost a waste of effort in being loyal as the YQ can be almost 2/3 rds of a paid fare.

Since you will be in UK, I'd suggest going with some American program as some of them have options to earn miles through non-flying means such as credit cards. Even AC might be good for you as they charge YQ only on AC flights.

The only exception for US carriers is DL as they also have some fancy charges for awards originating in Europe, plus they have few partners and have very very poor availability.