i5 vs i7 is a choice you need to make really. There's very little difference in most modern games. Many people have been using i5 CPUs bought in 2011 to game on quite happily for many years. It will probably be quite a while before games really take advantage of 6 cores (i5 8400), never mind 6 cores and 12 threads (i5 8700).
The i7 8700 might be more futureproof, but it costs a lot more, gives very little extra performance today, and nobody can known when and if it will have a real edge over the i5 8400.
As for white cases, the only two PCS generally have are the NZXT 240 and 410. I wouldn't recommend the 240 myself.
You can also send in your own case. For instance if you wanted PCS to use a white version of the NZXT 340 or the Fractal R5, you could buy one and send that in to them (if they agree, which I'm sure they will). Either of those are good cases.
After a lot of thought. I've decided to increase my budget to around the £1400 mark. Sorry for being annoying. Just cant make my mind up and want it to be perfect. Games im looking to play include,
Player Battleground Unknown
H1Z1
Destiny
Fortnite etc..
Because its a review machine, ie. it was reviewed by 'Custom PC' in thsi case.
Review machines are always cheaper than than if you configured it yourself, but they are only around for a limited time (though sometimes this can be for quite a while), and also you cannot change many of the components, so you cant't change the CPU, GPU, motherboard or RAM as a few examples.
A 1440p 144hz monitor will be a bit tricky for £450 max - they're usually £500 or more. You could borrow ~£50 from your keyboard and mouse budget to afford the better monitor. Get a KB+M bundle for £30-40 like the Cooler Master Devastator III