Generally speaking, I'd always suggest you'll see no more than 40% of what it cost you to build - that number could be substantially less with older tech.
Parting it out may be the friendliest to your wallet - and helps you understand what you've got there. Compare 'sold' eBay listings as an idea of the going rate for any one component. Example(s):
CPU - Looks like these sell more from the US than the UK, but going rate seems to be around £50 - you might do slightly better as you don't have to charge £20 shipping. Reasonable to call it £60.
Mobo - Seems like £35 is the going rate from completed sales
RAM - Maybe £20 if you're lucky
HDD - Honestly, if you're parting it out, just keep an HDD. They don't really have much resale value. Maybe £10
GPU - Looking at closed sales on eBay, you'd be doing well to get £100 out of it. Some go higher (£120) but many drop off anywhere from £80-£90 I'd say you could reasonably expect to see an £80 return - on a good day you should do better.
Case - Selling anywhere from £10-£25. Always er on the lower side, so lets call it £10
PSU - Another component with little to no resale value. A used PSU, especially one that's not even close to a 'top' PSU to begin with is a difficult sell. If somebody was to offer you £15 I'd recommend you take it. Otherwise, keep ahold of it. It never hurts to have a spare PSU on hand, even if it's not the best.
So, parting it out I would expect/hope you could recover around £200 while keeping the HDD & PSU.
Listing it as a complete rig, I'd suggest you aim to get £200 back (and assuming you have an OEM OS installed on the HDD, that will help sell the complete rig). There's also the added benefit that any potential buyer doesn't have to assemble - many people are willing to pay for that.
You might want to start at £250-£275, knowing you'll likely get haggled down a bit. Hopefully you can clear £200 or slightly more, but I wouldn't expect to see too much more than that.