While LHR is definitely "elite-heavy," I have received various "involuntary upgrades" at the gate from J to F (when still flown on my common routes) or even from E and M to J. This has happened both when I was Gold on QF and more recently as JQP.

If work or other circumstances dictated that I fly a carrier with another alliance, I would switch, but personally I'd stick to OW.

(And, really...do they think that I would kick and scream if they asked me to voluntarily upgrade at the gate? )

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.