Welcome! Sorry, I wish I could help you but I'm not from the UK.
However, I traded in my PSP this year aswell; a real waste of money.
I bought it for $250 and only got $100 back, so if you really want to make some money on it, sell it on eBay. There are probably quite a few people who have a PSP as a Want It Now item, so I would search through there.
There are quite a few people from the UK on this forum, so don't worry, someone will answer your question.
Nice to have you on.
*EDIT* You could probably try selling it here although I don't know if the administrator would get upset, but I don't think he would. Good luck.