Case: Sure.
Motherboard: I'm guessing you mean x79? Similar to the CPU below it's unnecessary.
CPU: No need for gaming at that price. Reduce to a 3770k (socket 1155) or 4770k (socket 1150).
Aftermarket CPU cooler: I know nothing of water cooling thus far so I'll leave someone else to inform you here.
Memory: I'm guessing you meant 2x8GB and not 8x8GB? :p That's fine.
Video card: Should have chosen a card with better cooling. If you go Crossfire be sure to become familiar with the third party tool "Radeon Pro" - it's been great for me at resolving frame rating/stutter issues.
Power supply: You'll definitely not need to upgrade that any time soon. For reference, I run two power hungry AMD 7xxx cards (7950 + 7970) in Crossfire plus several different drives on a 750W power supply and am still a bit below my watt limit. That sort of power supply should manage three cards just fine.
Operating system: Just get Windows 8 and use a program like Start8 to fix Metro. Metro-aside Windows 8 is superior to it's predecessor.
HDDs: Maybe a bit slow if you wanted to record with Dxtory, otherwise fine.
SS
Fine.
How are you making the total out at £1500? Should be considerably more than that for the parts you listed. /confused