Hiya, you can get a Ryzen 3 1300X based system, 8GB 3000MHz dual channel kit, ok psu, 1TB Seagate HDD, MSI A320M Pro-VH Plus, Aero cool case &
a Palit GT 1030 for about £420 + delivery costs.
Link to MB data
https://www.msi.com/Motherboard/A320M-PRO-VH-PLUS/Specification *check it carefully!
OcUk has Ryzen 3 1200 out of stock right now, that's why the 1300X. You would need to add £53 to get to the cheapest GTX 1050 that they have in stock right now.
You could save a little on the PSU but i picked this XFX
https://www.overclockers.co.uk/xfx-xt-series-500w-80-plus-bronze-power-supply-ca-022-xf.html
over the EVGA 450BT cause i find the cheaper EVGA's super noisy after awhile, like a year or 18 months. As you said your friend wanted to upgrade it over
time you would be better served by spending more on a good Gold rated PSU but i really understand the budget problems. Had them myself recently.
If you want to overclock the CPU then u need a B350 board which is more money....
Also i checked the clearance gpu section in case you wanted to go down that route but there is only 1 card under £100, that's an AMD R7 360 2GB for £90, i think
the GT 1030 has the legs on that (being much newer tech), but am not sure.
Wish i could do better for you, hope mickyflinn see's this thread, he really helped me out the other week.
Ahh 1 other thing! Phone Overclockers & ask them what they can come up with for £400 (if it is a hard limit) or maybe post in the customer service section.
Good luck.