I'd suggest taking a look at Kobalt's machines. There arent many 18" laptops so you may need to settle for a 17".
I'm an ex-Alienware customer, and despite some minor issues because they're a smaller company, and longer lead times ordering machines because of that, they're still giving me far an away better service than Alienware did, which is why I'm now ex-Alienware.
Unfortunately the UK market is a bit **** for competition in this area, you're either looking at Kobalt and the couple of other Clevo resellers, or Alienware, for a gaming laptop, with occasional interludes from the likes of Asus and MSI, WHEN thier kit reaches our shores and doesnt cost 2x what it should.