a ryzen 3600 will last you ages (most games only use around 4 threads and so a 12 thread cpu will have longevity). id go with 16 gb of 3600 MHz ram. personally id recommend a psu with a long warranty (as that's usually the first thing to go or reduce in power that then leads to your suspecting the the mobo or cpu is failing when in fact they are fine).
all mobo's have a m.s slot on them so go for a nvme ssd (id recommend 500gb at least, ideally 1 or 2 tb). I have the corsair mp510 and they are good value for performance
https://www.overclockers.co.uk/cors...d-state-drive-cssd-f960gbmp510-hd-065-cs.html
now the more difficult bit - what fps and resolution do you need/want? that determines which gpu and monitor to buy (the most expensive bits). The cheapest would be a 27" 1080 monitor (go for at least a refresh rate of 75hz) - and to pair that with a graphics card between £150 and £400 (say amd radeon RX590 up to a rx5700 or from NVidia a GTX1660super up to a 2060super). I tend to think that the AMD radeons are a little better value (but theres a saying "theres no bad graphic cards just bad graphic card prices" ie. a RX590 8gb is bad at £250 but is great when you can buy one at £170).
A £400 graphics card on a 1080 monitor will give you higher frame rates than a £200 graphics card - but you only really need that for fast paced shooters. Personally I get by with a lowish gpu on high res monitors (a 4gb fury on a 4k and a vega 56 on a 1440).
Personally I like a 1440 resolution monitor - and so you could get away with a £170 Radeon RX590 (I did use a Rx480 on my 4k monitor and so effectively the quicker RX590 would be fine on a 1080 or even a 1440 monitor).
I suggest something like
https://www.overclockers.co.uk/ocuk...zen-5-2600-rx-590-8gb-graphics-fs-1aq-og.html
but get them to put a ryzen 3600 in, pay more for 16gb of ram; get them to ditch the 2tb hdd and put a 1tb nvme ssd
as for a £300 monitor get more advice on that but £300 could get you a curved 1440 such as (which is failrly high up on the popularity)
https://www.overclockers.co.uk/sams...-gaming-widescreen-led-monitor-mo-23m-sa.html note that the monitor will probably be the part that you keep the longest
