Can't see how you can go wrong with OK+. Very reasonable qualification levels, 200% business class miles and free systemwide upgrades when flying OK Y.
The only drawback is the relatively low elite bonuses- but it's unlikely to make a big difference unless you are a very heavy ST flyer.
By the way, I can't see why you wouldn't fly AFKL out of the UK. If you are in London, there are a fair few nonstops from LCY; if you outside London, there are lots of feeder flights to both hubs, making the coverage noticeably better than BA/BD. There's also AZ and SU for flights "around" Europe (e.g. AZ for N. Africa/Middle East and SU for Russia/Turkey/CIS).

Oh, and if you do start flying more on ST, I'd recommend avoiding KL short-haul C (it's a mediocre economy product- they don't even bother calling it "business" anymore, opting for the nondescript Europe Select title instead).
CSA long-haul business isn't top notch, either, (prehistoric seats), but it's a good idea if you find a very good deal (I recall viewing flexible r/t fares for under £1000 on Europe-NYC) or get upgraded.