Hmm, that 16GB is probably the biggest dent in the budget - even at 2666MHz it's going to set you back £120, and if you're seriously considering Ryzen then really you want 2933 or 3200, which is another £10 - £15.
The cheapest B350 motherboard is the
ASRock AB350M @ £65. It does at least have a heatsink on the VRMs
Add the R5 1400 @ £150, go with the 2666MHz RAM, and you've got £165 left - just enough for the Sapphire RX 570 PULSE ITX.
IMO that's the best bang for buck you're going to get at £500
if you're set on 16GB of RAM - personally I'd be tempted to drop to 8GB of 3200MHz RAM (£80). The £40 saving would let you pick up a 4GB RX 580 and have a tenner left over.