United or Lufthansa are excellent. United has connections to most US destinations, and Lufthansa to the rest of the world. You can also fly on SAS or British Midland.

The only drawback with UA, is you don't get elite bonuses on SA airlines. (You may get them on Lufthansa, but if so this is new.)

If you are going to be in the UK for many years, you might also consider British Midland. They serve most European locations and you can get credit for other Star Alliance flights. Plus, you get the elite bonuses and upgrades when you fly on them.

On the BA thing, another option is to go with American. You get credit for all BA flights except the transatlantic one. For these, you need to fly american to get credit. I also think you get elite bonus miles for flying BA and using AA points.

Remember that flights are usually very short in Europe, and they don't have "American" first class. Their first class is basically a better meal and assigned seating. When flying economy in Europe you are crammed into the back of the bus. So the real bang for your points is the transatlantic travel.

When I lived in London I collected points on United and flew them almost exclusively on trips to the US. I then flew SAS, Lufthansa and bmi quite a bit to the continent for business. I also flew BA a bit, but only when I had to. For pleasure travel, I flew mostly on discount carriers like Buzz, Virgin, and RyanAir.

Good luck.