Buy used parts, or used builds. Find person, who can check parts, or check it by yourself, if you know what you are doing.
There is a lot of good cpus for money.
Look for 2nd, 3rd or 4th gen intel i7. 1st, 2nd gen ryzen 3, 5. Maybe 7. Look at some older lga2011 motherboards, and cpus. It have more cores, more threats, more cache, posible to overclock.
Also there is a lot of good gpus. I think the best gpu for money rx570 which competes with gtx 1060 3 and 6gb in some scenarios. Also check gtx1060 6gb/rx580.
Look some videos in youtube, like tech yes city. Check aliexpress for parts(it takes longer time for delivery, but cheaper).
I know it's hard to get pc for 300 pounds, or 450$, but possible. Aim 200$ for cpu+mb+ram+(better cpu cooler if overclock). And 200$ for case, psu, gpu, hdd. I'll recommend you invest in better cpu, and save for better gpu later, or vice versa.