By choice I use Fieldworks near London Bridge for recruiting participants for user testing or research.
I’ve used them for years and they typically respond well to a brief and keep me well informed of how recruitment is progressing.
Other people have mentioned to me that they use:
- Criteria in Hampstead
- Focus 4 People in Elstree
- Indiefield in Enfield
- Saros near Oxford Circus
Let me know how you get on with any of them.