Looks like they're going for about £175/180 in MM. Bargain compared to a new card. A 1070 for £220 would give a bit more grunt, but if budget was really tight it's not worth the extra £40. 1080ti going for £550 in MM. Seems crazy to pay nearly triple for a card that doesn't offer double the performance.
Thing is the real threshold to look at is: res + quality + minimum acceptable framerate. So let's say you want the game you really look forward to play to perform better at 4K & min 30 fps & High-V High mixed preset. Then you'd have to look at what can deliver that and how consistently.