Based on the cost of a self-build, the price seems fair (I don't think OCUK sell single 8GB sticks), but I am no expert on budget pre-builds.
AMD Ryzen 5 4500 Six Core 4.1GHz (Socket AM4) Processor - Retail - £79.99
ASUS PRIME A520M-A (Socket AM4) DDR4 mATX Motherboard - £71.99
Team Group Vulcan Z T-Force 16GB (2x8GB) DDR4 PC4-25600C16 3200MHz Dual Channel Kit - Grey - £44.99
Asus Radeon RX 6500 XT Dual OC 4GB GDDR6 PCI-Express Graphics Card - £149.99
Kingston A400 120GB SATA 6Gb/s 2.5" Solid State Hard Drive - (SA400S37/120G) - £22.99
Aerocool Tomahawk Midi-Tower ATX Case - Black - £29.99
Kolink Modular Power 500W 80 Plus Bronze Modular Power Supply - £29.98
Grand Total: £441.91 (includes delivery at £11.99)
I must say though, you will lose a lot of performance from having a PCI-E 3.0 board with a 6500 XT and using such slow single channel memory.
What are you using the PC for? If you won't be gaming, I'd go for a 4600G or 5600G based pre-build, rather than the 4500 (which has no integrated graphics). The £150 on the GPU could be far better invested elsewhere if there's no gaming.