I've heard Overclockers will put together your own build for about £100 extra if you email them with the parts from their site, and that would be your best option if true. This one would do a job, but of course you could do a lot better getting someone on here to pick parts for you if you have a friend or someone who could put it together for you for a fee maybe?
https://www.ukgamingcomputers.co.uk/hex-x6-gaming-pc
I'd bump the RAM up to 3200mhz using their configure option, even though you will have to pay extra for silly RGB lights. Not sure if you need an operating system?
Also, there's this from PcSpecialist. The case is a bit cheap and I don't know much about it. The 3300X is excellent value. Motherboard is kind of basic but good enough for what you need. GPU is substantially better here:
Manufacturer of performance custom computers and laptops. Customise your ideal gaming PC or laptop using our advanced configurator.
www.pcspecialist.co.uk
Case
PCS SPECTRUM RGB MID TOWER CASE
Processor (CPU)
AMD Ryzen 3 3300X Quad Core CPU (3.8GHz-4.3GHz/18MB CACHE/AM4)
Motherboard
ASUS® PRIME B450-PLUS (DDR4, USB 3.1, 6Gb/s) - RGB Ready!
Memory (RAM)
16GB Corsair VENGEANCE DDR4 3200MHz (2 x 8GB)
Graphics Card
8GB AMD RADEON™ RX 5700 XT - HDMI, DP - DX® 12
1st Storage Drive
250GB SEAGATE BARRACUDA 120 2.5" SSD, (up to 560MB/sR | 540MB/sW)
2nd Storage Drive
1TB SEAGATE BARRACUDA SATA-III 3.5" HDD, 6GB/s, 7200RPM, 64MB CACHE
Power Supply
CORSAIR 650W CV SERIES™ CV-650 POWER SUPPLY
£973.00
inc VAT and Delivery
There's a few things you could change about but it's not a bad build for the price. Better than the UkGaming one IMO.