This one https://www.amazon.co.uk/Lenovo-GTX-750-Certified-Refurbished/dp/B00TG06LP8/ref=sr_1_16?s=electronics&srs=8362590031&ie=UTF8&qid=1481539122&sr=1-16 would have been good value for money and would get the job done. but sadly, the form factor means upgrading it is gonna be very cumbersome. upgrading the RAM isn’t a problem, but if your son wants to upgrade the video card which would be necessary for working on larger projects, you will be limited to half size graphics cards which are both rarer and less powerful. a few months back my main rig with a GTX 1080 had its motherboard die which forced me to fall back to my secondary rig with a GTX 750 ti. UE4 ran well, but i did miss my 1080. then again, i’m very demanding when it comes to graphical performance.
the second one https://www.amazon.co.uk/CCL-Storm-200-Gaming-PC/dp/B01MTP37ED/ref=pd_sbs_147_8?_encoding=UTF8&refRID=YEWJEKXC1HA93BTN9684&th=1 looks more interesting. the model £479 model you linked to lacks a discrete graphics card. i recommend you buy a GTX 1050 ti to go with it like this one https://www.amazon.co.uk/Palit-GeForce-1050Ti-StormX-Graphics/dp/B01M2ZC61B/ref=sr_1_1?s=computers&ie=UTF8&qid=1481542212&sr=1-1&keywords=1050+ti, that inbuilt gpu is kinda ****. that should put you back about £600, but i strongly believe it’s worth it