Ops my bad
On mouse I wouldn't skim as crucial to have precise mouse when playing fps, as the keyboard could be any as it doesn't help nearly as much as good quality mouse. Personally I would buy this mouse from rainforest for 51 pounds "Logitech G502 Proteus Core Tunable Gaming Mouse" It would be useful if you could measure your stepson's hand.
You can also save on monitor by buying this lg ultrawide 25 inch. It's roughly chepaer by 15 pounds. The only issue with that is if game doesnt support resolution of 2560x1080 you will play with black bar on the sides. Other than that it's similar monitor to asus. Although asus have speakers compare to lg that doesn't and is build of plastic compare to asus aluminium.
https://www.overclockers.co.uk/lg-25um57-25-2560x1080-ips-gaming-super-wide-widescreen-led-monitor-black-mo-135-lg.html
For keyboard anything will work, although mechanical keyboards just feel so nice when typing or gaming. However they can be little expensive starting from around 75 pounds for Corsair CH-9000040-UK Vengeance K65 Compact Performance Mechanical Gaming Keyboard up to 150 pounds. Also for mechanical you got different switches. More on this in this video
https://www.youtube.com/watch?v=PXCKHoiNu1o
So let me know what keyboard would you like membrane or mechanical and then I can help you decide.
Lastly you can also buy cheaper case.
https://www.overclockers.co.uk/phanteks-enthoo-pro-mid-tower-case-with-window-black-ca-002-pt.html
This one costs 34.99 pounds cheaper and is made of steal and plastic whereas the one that I recommended first evolv full atx case is made fully of aluminium so it's really up to you, which one you pick. You can even get any other case just make sure that it supports full atx motherboard and make sure cpu fits (in terms of hight but hyper 212 should fit in the most of the case). I wouldnt skim on case either as if you are going to see your case everyday you dont want to look at something you don't like, for example my first case nzxt phantom red I really hated the colour of it as it was cherry colour not bright red and I couldn't return it as got it from auction on bay, 3 months later I replaced it with storm trooper from cooler master so at the end I spend more money than originally planed. Hope this helps
EDIT
1tb WD blue hard drive is roughly 42 pounds here.
SSD will be nice upgrade but you will be going way over you budget. It will only improve the speed of turning on and off and application load up time installed on the ssd. eg typical boot up of hdd is 1 minute compare to ssd around 20seconds
