OP,
Are you going to be buying expensive or cheap fares? If cheap your redeamable miles earning will be very low, 5 miles per $ spent with no status then 7 for silver and 8 for gold. A cheap international ticket from UK to states, you could earn a lot less miles than you flew quite easily. If you are buying cheap fares you might be better off getting a points earning credit card as suggested above then buy your tickets through that portal (make sure not bulk tickets) then even on your award flights you will earn miles for status.
But if buying expensive fares then stick with UA