Currys/PC world do not build their own they only act as sellers for Lenovo, HP, Acer, Asus etc etc
If you know what you are looking for where you actually buy the PC is pretty moot (it all hinges on price/customer service/value adds).
Not bought a pre-built in years[1]so I couldn't say which company has a good rep for systems, also unable to tell you good retailers local to you as I am in Scotand not London, but still not as far as North America.
[1] Sort of told a lie there No2 son (23 so autonomous but still at home) got a Lenovo all in one from John Lewis, maybe because they are a co-op rather than a business their service seemed first rate. Has been one problem, JL checked it wasn't something obvious and had unit sent to Lenovo for the actual fix.