I have my buddy’s old “top of the line” gaming rig. I cost him about $2500 and I picked it up for $50 (tower only, no drives) three years later. I got another 3-year-old AMD system for $220. For something only half that age, you will likely see a better return than that, but it shows you how quickly the value drops.
I think @Sarcasm has a point that it may be worth more as parts, but bear in mind that the sort of person who assembles their own PC in the first place (in other words, people who actually buy parts) either want something better than last year’s model or are looking for a bargain. Between that and the fact that you’ll have a hard time selling all of the parts that way, you’ll be quite lucky to see £1000.
BTW, what do you think it’s worth? Not how much you paid, but an honest estimate of it’s current value.