Personally speaking ............
I'd definitely buy Hewlett Packard, and I'd buy it from Staples, excellent customer service and the staff seem to know their stuff, I'd avoid PC World/Dixons/Currys, I worked for them briefly, they are absolutely wank at customer service, they are there to sell sell sell, regardless of what you want or need.
Staples quite often have end of line deals or shop models, that come with the full warranty.
I'm writing this on a HP desktop PC, looking at a HP LCD monitor, both bought from Staples, honestly you won't go wrong