www.biotrax.com
These guys do mystery shopping aswell as drug trials of which you are not expected to take part unless you so wish.
They do charge a one off membership but you will easily get that back on your first shop and a lot of there work is London based.
You could also join YouGov
http://www.yougov.com/users/registrationIntro. asp?jID=3&sID=2&UID=
That pays 50p per online survey, so it's just a fun thing really.
I am a member of Biotrax for drug trials myself.
Good Luck!!!!!
B. :-)