If I were you I would sell your Pro as it was out of the box. Products really lose their value once customized.
Sure it has faster RAM now, additional storage, and optical drive removed?
All 3rd party products though. It would be better to sell it the way Apple sold it to you.
You'd be better off taking out these custom parts and selling them individually, where you are likely to get more value. Or, keep it for later.
Sure there are exceptions - if you find a tech savy guy with this kind of need. But usually people want an apple laptop the way apple made it.
Plus, seems like Amazon.UK is selling used 2012 Pros for 700pounds. Should give it a look.
Good luck, and I hope you end up getting what you want for it.
PS: Damn, mac prices are really inflated in the UK