I'd stay away from prebuilts personally, but given AMD's announcement about the new CPUs being incompatible with thhe B450 motherboards, it might be worth waiting for the new motherboards come out, as there should be people who buy them to replace their current B450 motherboards. Also given price to performance, I'd probably go for an AMD CPU, possibly the ryzen 5 3600. I suggest a ryzen also due to its included WRAITH cpu cooler which performs well so you dont need to worry about any cooling issues.
You can find many B450M MicroATX boards on ebay right now around the £40-70 price mark. Not fancy but will do the job
CPU+ CPU Cooler = £155
RAM = £60 - Get any DDR4 8GB stick
Motherboard = B450M AM4 Socket motherboard
Case = Get any case that comes with case fans ( I have the Corsair 275R, comes with 2 fans that are perfectly sufficient for cooling on their own. (£50))
With the remaining money you should be able to get something like an GTX 1660 Super brand new, but if you're willing to risk it then you can try for a higher end one on ebay. I have a AMD Radeon Sapphire R9 380 and the 1660 super gets around x2 better benchmark scores. I run GTA V and OMSI fairly well on this, GTA topping 63fps and OMSI stable at around 30-40 FPS.
If my maths is right, that should be around £530 or something like that, you're going to want a copy of windows but iirc you can still convert a windows 7 licence. (Don't quote me on that)
One final note, I wrote this assuming you already have a power supply of some sort, if you dont, then prepare for it to cost a good £60 more.