I lived in the UK about a year ago. I maintained my status in United MP, and they actually had a few promotions targeted at us in the UK (10K minimum for a roundtrip between US and Europe, Free RCC membership).

If you are going to be travelling back and forth to the US quite a bit I would recommend you stay with UA. Another upside is that you can fly on SAS, Lufthansa and British Midland and get MP miles/status and use their lounges. The down side is you don't get bonus miles for these flights.

If you will be doing purely European travel, I recommend you pick a british airline so you will get status, bonuses and upgrades through them. Upgrades in Europe though aren't as good as the US. A business class flight is the same seat as coach (maybe slightly bigger) with a better meal. You get a meal and free drinks on ALL flights in Europe though. I once took a 28 miles flight on CrossAir and got chocolates and a drink!!

As posted previosly, BA and BMI will have more promotions targeted at UK residents. I don't remember as many ties-ins though to phones and groceries, etc. BA also has some great specials and flies everywhere in the world. Its not worth much though if you return to the US (except a trip to the UK). British Midland offers good fares to Europe, and now a few flights to the US, but you need to go through Manchester. There are also a plethora of discount airlines that you can use for a weekend jaunt. Check out RyanAir or EasyJet.

Have fun in the UK. Feel free to email if you want any other tips.

Chris