the simple answer would be " a domain lookup script "

WHMCS includes domain lookup through the whois services as part of the system , in addition to that you'll need one of more *registrars* to actually register the domains through
for .uk you'll want to use Nominet, or one of their registration-agents, or one of their reseller who provide an API to do the actual regsitrations, as Nominet "run" the whole of .uk
for .tv you'll want to use Enom or one of their resellers, as they run the Tuvalu Registration System
for .com there are various companies
for .ie you'll need to use IEDR or one of their resellers, as they run the .ie ccTLD
etc - most poeple dont want the level of overall "control" we do, and deal with just a few larger registration companies that themseleves have arrangements with the registries.
The one that works best with whmcs and some of the other billing systems is enom, closely followed by netearthone and a few others - you can "pick and chose" for each type of domain you want to offer, so if one provides as serice or facility that another doesnt but they only register 10 of the 270 domains types you want, then you can use them just for that 10 etc
