Hi fireside, well the only way you'd have to spend £1200 is if you bought graphics cards at the moment, for that figure normally you'd be able to build a PC that could game on a 4k monitor, as opposed to a 1080p monitor you'd normally use.
As a rule you want to spend as much on a graphics card as possible, around 40-50%, but ist important to remember that unlike other components, its VERY easy to upgrade, whereas to upgrade the cpu, may mean you have to build a whole new system.
In regards to buying separately, this all depends who brave you want to be putting things together. PCs are basically like adult electronic lego, it's a bit scary before, but once you've done it it's actually quite fun ( in an extremely needy way!) So go on YouTube and look up PC build guides, in particular 'linus tech tips'.
In regards to price, I bought my graphics card before shortages for £110, they are now selling for £450!!! Because of their you shouldn't bother until they go down, just too much money, in my opinion.