if you want to get serious then dont make the mistake i made of buying innovatek.... you'll only end up changing it all. IMO innovatek kits are only good for someone who wants a simple setup that does a fair job and they can forget about it.
If i were you i woudl bite the bullet and get some stuff DHL'd over from Dtekcustoms... quite conveniently they are supplying the whitewater block in a few weeks time as well.
Failing that, have a look at
http://www.wizarddesigns.co.uk/ The double rad there is a heavyweightr performer and the cuplex evo block is pretty decent as well although not up with the best.
Apart from that, sourcing insode teh uk you are pretty limited. Theres some real daft kits out there but the most accomplished seem to be the innovatke and danger den based systems. Whilst the Eheim pumps are as good as any, none of the DD or Innovake blocks (including GPU) are up with the best and the innovatek is also a slow flowing system which will never be super-high performance. As for rads, BIX appears reasonably good but again, not a patch on a heater core.
I had some chap grab the heater core from a Ford Sierra the other week... it was a big awkward shape for my liking but maybe you could consider that route for your rad ... it'll only cost you 10 quid or so.