The point about an alliance is that it shouldn't matter. Most airlines fly to LHR, so you should have a good *A option (for example) to any destination of choice. If you want to fly to the USA, fly United. To Asia, SQ or LH via FRA, or many other options.

You make a good point about WT+ though - there are not many other competing products other than Virgin. And even then you can earn destination (not status though) miles on bmi.