Whatever you do - please do not get a used car seat from someone you don't know. You never know if it had been in an accident - and even an accident at a low speed can damage the carseat so that it does not protect the child as adequately in a future crash.
Also, I don't know UK brands, but if you do get a used car seat from a friend or family member make sure it hasn't expired (yes, car seats expire!) the follow US brands state their car seats can be used for the following number of years:
Britax - 6 years
Eddie Bauer/Safety 1st - 5-7 years
Evenflo - 6 years
Fisher Price - 8 years
Graco/Century - 6 years