if you don't want to build it yourself but more choose what you want in it i'd hop on alienware, mine personally cost about double what your prices are but that was cause i needed it to run a lot of demanding programs and very demanding games, it really depends on what you're using it for, if you have 800 - 1000 quid then it may be better to look into what you use most and tailor it to work around your most commonly used programs and if the final price comes to only 600 then just throw in a nice monitor, or even do i dare mention, save it
also a tip on price saving, if you end up buying individual hardware don't buy it all from one place, i know that a lot of places say if you spend £xxx amount then get off £xx but you could easily save more if you just look around and there are a lot of good stores that'll sell for good hardware for a fraction of what they cost from the source, however i may recommend getting your motherboard and graphics card from trusted place e.g. nvidia, things that cost a lot and can overheat may well be in better condition if you buy it from the source and may well have a longer warantee on them, my motherboard died about 6 months ago, turned out it was an issue with the hardwares assembly but i gout it free because i was in warantee, however if i hadn't had warantee it'd of probably cost me something like £800 - £1000 when you account for engineers being paid, by you, for every hour of work, that total can really rack up. i may also recommend getting yourself a liquid cooling system instead of fans, its expensive but effective, wish i'd bought one, hope this helps
