You are looking at around 860 watts (GPUs and CPU at stock speeds) you'd need to consider dissipating with radiators. Overclocking components raises TDP, so you'd want to plan for that additional heat accordingly. You are likely looking at around 1000 watts when it's all said and done. You are going to need at minimum, two 3x120 rads or similar cooling potential.
1000 just for the water loop? What UK sites are you looking at? Maybe we can help point out what you'll need.
As for watts...I am speaking in terms of heat in watts produced by the CPU and GPUs...not the power required to run them...BUT...there is an almost direct correlation to the power needs and heat output...watts are watts. Power usage watts do not mean a 1:1 in heat watts produced, but it's usually within 75-80% or so...enough to effectively estimate the amount of cooling specific hardware will require.
are you set with the haf x case? if not the corsair 800d is a good case for water cooling. It has holes predrilled up top to allow mounting a 3 fan 360 cooler which will be able to pump out most of your heat. you can also mod the bottom of the case to fit a 2 fan cooler which should be enough for your build i believe. check out the build log in the watercooling forum here. you will find alot of setups that people have made, some of which are very similar to what you are going for. some of them even show how they modded their case to fit more rads in.