You are not clear where that motherboard / CPU comes from. Did you buy one cooling system consisting of a watercooler and various other parts, including a motherboard and CPU? In that case, since you don't care what parts that system is made of, just that it works, the cooling system that you bought needs to be fixed.
On the other hand, if your PC was running in the same room as the cooling system and water from the leak dripped onto your PC and damaged it, I suspect that they are not responsible unless the leak was caused through gross negligence (but this is just an opinion).
But in that second case, your home insurance will probably cover the damage to the PC. Not the damage to the water cooler.
Edit: I had got the complete wrong impression what a "water cooler" was - I had thought it was some part of a home cooling system that keeps your home cool in a hot summer :-) Got it now...
Tricky question this. Your water cooler is clearly designed to be put into a computer, and it is quite obvious that a leaking water cooler will inevitably create awful damage to your computer. In other words, this kind of damage when the water cooler is used as intended and breaks is inevitable.
Therefore I would think that a company making such a cooler must take outmost care that it cannot possibly leak. "Not leaking" would be ten times more important than for example "working". Like buying a power supply; the priorities for the maker would be "doesn't kill anyone", "doesn't destroy the computer", "powers the computer", in that order.
I think what happens is outside consumer protection law, but damaging your property through negligence is a tort and they will be liable, because in your case there is a very strong connection between the problem and the inevitability of damage. On the other hand, if you bought a defective vase, and it broke and the water from the vase destroyed your computer, that would much more likely just tough luck (of course they would have to replace the vase).
You probably should first tell the seller and the manufacturer that the computer damage is directly caused by their water cooler, and they should have never sold a water cooler that could leak, and they should pay for the damage. If they refuse to pay up, you'd need to go to small claims court, citizen's advice bureau, or take a lawyer.