Hi fellow London based traveler :-)

How hard would it be for you to stretch to 50k miles flown?

Getting to *Gold (vs silver) is a big deal if you mostly fly in economy, because it means you can use the lounges. (A great benefit at IST ^) Gold also gets treated very well on TK and AC (first boarding group, with business, plus use of the business class check in) and much better than silver on UA.

If you can get to 50k then you can get Gold on United. If you can't then look into Miles&Smiles (TK) and Agean (A3).

Check fare codes carefully. United is actually pretty good and even cheap tickets (at least G and above on Turkish and all United fares) credit at 100% which may not be the case for A3's and TK's mileage programs.

I would also read about redemption on the specific airline's forums. I know it's a pain for Miles&Smiles and may involve trips to Heathrow.

Finally - cross-alliance upgrades are hard to do. You need an expensive ticket (Y or B) plus 10K miles to upgrade. There is one loophole here - if you buy a TK Comfort Class ticket, and any segments aren't available as Comfort Class (e.g. LHR-IST) it books into full Y so you can upgrade that segment of the trip to business for 10k miles.

On the rare occasions they have to do it then it usually goes:

Airline's elites.
*A elites.
Everyone else.

I have been OpUped internationally three times in 400k miles of flying so I wouldn't worry about it when picking a FFP. (2 on CO (CO Silver, CO Gold), 1 on AC (CO Plat/*Gold).)

UA is good for straight up redemptions, not upgrades. If you want to upgrade then go look at BA's WT+ to CW upgrade chart which has some fantastic loopholes in it (calculate the net miles required UK-USA as a BA Silver card holder for example).

Also consider that upgrading any flight ex-UK will cost you an extra £75 in APD.