In fact me too I would stay with OW.
Ehi, you are now located near the European heart of OW and I think you could easily benefit of it redeeming awards on many routes and enjoying the best of their services in LHR.

*A is very good but you need to travel a lot towards continental Europe with transfers in BRU, FRA, CPH (just to say the nearest hubs) in order to see the difference.

On the other hand ST may be useful if you have to go often towards France (CDG), Italy (FCO) and yes...AMS.

Personally I would prefer the alliance that better serve my area of reference (number of connections, lounges, fast track...) and, usually, is the airline based in the country where you live....with some notable exceptions (like for eg Milan which is better served by *A than ST)...but London is not one of them.