In mid 2017, I got a new Dell, with Windows 10 Pro license, and full warranty, for about $450 plus tax. There was a sale, to be sure. Also:
- 8GB of 2133 RAM instead of 16GB of 2666 RAM
- Basic GT 730 video card
- 1 TB HDD but no SSD
- Standard Dell OEM 460W PSU, but which they claimed (and they would have to deal with warranty issues) would support a 225W GPU
Now, you have an aftermarket PSU - but I think you have the pre-2017 redesign of the Corsair CX, though that's just a guess on my part. You also have the greater amount of RAM, but RAM prices have since come down significantly. The 1050Ti is decent as a 1080p card for its time, but not worth what it once was.
I don't know if your Windows license would transfer over with the PC (ie: it's an OEM license rather than a retail license which you'd keep to reuse on a new system).
I don't know what the used PC market is like in the UK, but in the US, I imagine it would be difficult to get more than USD$300 for the whole thing.
Go in my sig and click on the link (open the spoilers first) for ChromaTron. That's what was possible with under $600, though, admittedly, pre-Christmas sales were involved in some of the prices. Also, the video card was a carry-over from a previous system.