(Even if you pay "somebody" 250 pounds to put it together, i think it is still worth it....but that's just my opinion.)
I'll do it for £200 or three bottles of decent red wine (my usual fee).
The one that catches my eye are storm force from Argos , they seem to offer decent specs for the price which seems a little to good to be true.
You gets what you pays for. The specs of most pre-builts rarely include the exact manufacturer and model number of the motherboard, graphics card, system RAM and most importantly the power supply.
The result is you'll probably get a low-end (cheap) motherboard from any one of three or four different manufacturers, depending on what's available at the time of manufacture. It should work fine, but might not have enough USB ports for your requirements.
The PSU is the sticking point. It's where pre-builts save most money. A good PSU is expensive (often £90+) and comes with a 7, 10 or 12 year warranty. A junk PSU is cheap (under £40) and may have a 3 year warranty or less. Junk PSUs have a tendency to stop working or go bang after a few years, which can be bad news for your mobo.
Check the amount of VRAM (Video memory) on the GPU card. For gaming 8GB is the absolute minimum, 12GB is better, but some titles require 16GB VRAM. Look for 32GB system RAM (not the same as VRAM). The SSD drive should be at least 1TB, but for lots of games, you'll need a 2TB or 4TB M.2 NVMe SSD. If you need more storage at a later date, you'll need a mobo/case with room for more drives. A cheap pre-built may have very few upgrade options.
I haven't bought a pre-built in decades, but I accept the fact some people are nervous of building a PC with zero knowledge. As a qualified electronics design engineer, I prefer to build my own systems.