The following are some sources of used amateur (ham) radio equipment in the UK. I am not endorsing any of them specifically, just putting the links here for information.
Commercial DealersMartin Lynch & Sons
Waters and Stanton
Mainly CB radios but sometimes have used amateur radio equipment for sale.
Buying privately means buying without any sort of warranty, but prices will be lower and there is no come back. Ebay prices can be as high as buying from a dealer so be careful.
Ideal for finding items for sale locally.
G3CWI's Amazing Online Flea Market
Amateur Radio Sales
JunkSale Amateur Radio
Amateur Radio Equipment for Sale or Trade