There are a number of companies out there who specialise in building gaming computers. Google it - and then check out their reviews. (Chillblast have a good reputation.) My son just bought online from one of them - I forget who - and has been very happy with his PC. He wanted a spec good enough to allow him to use the PC for VR games as well as give him good performance on demanding games. He chose an Intel Core i5 processor, 8GBytes of memory and a GTX1070 graphics card. For gaming, those are the three key things - everything else is pretty standard. An SSD may give valuable additional responsiveness. Pay more for more memory on the graphics card - it's worth it. Some say it can be worth paying extra for a brand name (rather than generic/no name) power supply, CPU cooler, case and so on. You should be able to get him a very acceptable PC for his purposes between £1,000 and £1,500: more than £1,500 will probably be overkill, less than £1,000 and the PC may not be up to the job.
This is an example of the kind of thing (if you don't mind the garish case...): click here