Recommendation for consumer research company in UK

The first question is, how credible are consumer research companies?
My inner cynic says they can very easily just make up the data and I have no way of actually validating it.

What companies would you recommend, I am interested primarily in the UK market for now.

Honestly, I am considering just going myself to Leicester Square or Hyde Park and start surveying people myself but If I can get the same for a few hundred pounds then I would rather pay and use my time for other work.