Don't use Pimlico Plumbers or Aspect Maintenance.
They are number 1 and 2 in terms of size in London and considered the 'gold standard' of builders here, but I've just won a legal action against the latter (finding out they had totally inadequate public liability provision in the process); and I have been ripped off and let down regularly by the former (my insurance company specified them - probably because of retainer agreements or some such).
Ratedpeople, Problemsolved and Mybuilder are full of fake reviews, no doubt left by friends of the builders and trades - it's very difficult to get lucky with someone good. It's certainly a truism, in London at least, that good builders don't advertise. Certainly don't use anyone putting a flyer through your door.
I'm afraid word-of-mouth is really the only way. I don't just ask friends - I drive around watching building projects in my neighbourhood, and ask the neighbour for feedback once they're completed. Architects are also a good source of builders, even if you're just phoning around and have no plans to engage an architect.
In the recent past I've been impressed by Frank Neale in Shepherd's Bush and Ark Construction in Wembley, but note that they are expensive and - no matter how much you pay - even the best builder only really gets an 8/10 or so. The sums of money involved are so huge and the expectations so great that you will never really feel truly positive about your builder, no matter how successful the end result.