You have ~£200 for a GPU thereabout, probably worth getting a 2nd hand Vega 56 if you can. They are going for around £180 atm.
If you want a 5700 XT you're pushing the budget to ~£1000 region. Or ~£945 with a standard 5700 GPU.
Here's an initial spec idea to start you off (excluding GPU ofc)...
My basket at Overclockers UK:
- 1 x AMD Ryzen 5 3600 Six Core 4.2GHz (Socket AM4) Processor - Retail= £188.99
- 1 x MSI B450 TOMAHAWK MAX (Socket AM4) DDR4 ATX Motherboard= £106.99
- 1 x Team Group Vulcan T-Force 16GB (2x8GB) DDR4 PC4-25600C16 3200MHz Dual Channel Kit - Black/Grey (TLGD= £89.99
- 1 x NZXT H500 Midi Tower Gaming Case - White Window= £69.95
- 1 x Corsair Force MP510 series 480GB NVMe PCIe M.2 Solid State Drive (CSSD-F480GBMP510)= £79.99
- 1 x Corsair RM Series RM650 650W '80 Plus Gold' Modular Power Supply (CP-9020194-UK)= £76.99