Isaac2005

Here's some changes I made.
You get gold efficiency psu, it's semi modular but you have to connect the 24pin atx cable and the video card 8 pin connectors anyway.
You get 512 GB and nvme  SSD (higher speed) ... and you get to use the M.2 connector you have on the board.
The motherboard is fine, I was about to suggest MSI B450M Gaming Plus but this Asrock model is decent as well. You're really limiting your choices with mATX - if you go with ATX case, you may get better quality motherboards at the same price, but the case may be a bit more expensive.
 
I would encourage you to drop the 2 TB mechanical drive and use that money in the monitor budget - it's really a shame to use that powerful video card with 1080p and a 21.5" monitor.
For example, I'd suggest spending 150 uk pounds for :
LG 25UM58-P 25.0" 2560x1080 60 Hz Monitor (25UM58-P) - PCPartPicker
or 160 uk pounds for :
LG 25UM58 25.0" 2560x1080 75 Hz Monitor (25UM58) - PCPartPicker
 
Another great option is  this at 190 uk pounds AOC Q3279VWFD8 31.5" IPS LED QHD (2560x1440) Freesync 75Hz monitor. (VGA, DVI, HDMI, DisplayPort)
In the build below, there's 44 uk pounds left in the 900 budget, and if you drop the 51 uk pound mechanical drive, you have 95 uk pounds. If you take out the 80 uk pounds monitor, you now have 95+80 = 175 uk pounds, so you'd only go above your budget by 15 uk pounds to get a decent big monitor.
Mechanical drives you can buy any time of day, but you don't change the monitor so often, buy something you don't have to strain your eyes on.... 21.5" in 2019... bah.
 
PCPartPicker Part List Type Item Price CPU AMD Ryzen 5 2600 3.4 GHz 6-Core Processor £108.00 @ Amazon UK Thermal Compound Arctic Silver 5 High-Density Polysynthetic Silver 3.5 g Thermal Paste £5.98 @ Amazon UK Motherboard ASRock B450M Pro4-F Micro ATX AM4 Motherboard £64.13 @ CCL Computers Memory Patriot Viper 4 16 GB (2 x 8 GB) DDR4-3200 Memory £63.91 @ CCL Computers Storage ADATA XPG SX6000 Pro 512 GB M.2-2280 NVME Solid State Drive £56.21 @ CCL Computers Storage Seagate Barracuda Compute 2 TB 3.5" 7200RPM Internal Hard Drive £51.24 @ Amazon UK Video Card Sapphire Radeon RX 5700 8 GB PULSE Video Card £340.65 @ CCL Computers Case Aerocool QS-240 MicroATX Mid Tower Case £31.99 @ Amazon UK Power Supply Corsair TXM Gold 550 W 80+ Gold Certified Semi-modular ATX Power Supply £63.97 @ Amazon UK Monitor AOC G2260VWQ6 21.5" 1920x1080 75 Hz Monitor £79.97 @ Laptops Direct   Prices include shipping, taxes, rebates, and discounts     Total £866.05   Generated by PCPartPicker 2019-11-19 20:54 GMT+0000    

If you play a lot of FPS games, switch out the 60Hz monitor for a 144Hz one.

I can't give a specific number, but in kerbal this PC would run super well

PCPartPicker Part List Type Item Price CPU AMD Ryzen 5 2600 3.4 GHz 6-Core Processor £106.38 @ Aria PC Motherboard ASRock B450M Pro4-F Micro ATX AM4 Motherboard £65.98 @ Amazon UK Memory Patriot Viper 4 16 GB (2 x 8 GB) DDR4-3200 Memory £54.99 @ Amazon UK Storage TCSunBow X3 240 GB 2.5" Solid State Drive £23.99 @ Amazon UK Video Card Sapphire Radeon RX 5700 8 GB PULSE Video Card £345.85 @ CCL Computers Case Thermaltake Versa H15 MicroATX Mid Tower Case £29.99 @ Amazon UK Power Supply Corsair Vengeance 500 W 80+ Bronze Certified ATX Power Supply £51.90 @ Amazon UK   Prices include shipping, taxes, rebates, and discounts     Total £679.08   Generated by PCPartPicker 2019-11-12 16:09 GMT+0000   I left a lot of room for tax and shipping, as well as some extra space to upgrade to more storage if you like.