Nominet have strict rules on use of .ltd.uk domains, and an algorithm for their generation
The only two possibilities for Wiki UK Ltd are wikiuk.ltd.uk or wiki-uk.ltd.uk; anything else fails their tests. As the intention is to trade as Wikimedia UK that would probably not be advisable as it could cause confusion amongst potential donors.
Additionally as the .ltd.uk second level is not as widely used, public awareness of it is very low. Use of less popular domains tends to cause problem with people "filling in the blanks". Plenty of people complain to me that they get a bounce message when emailling me, only for me to discover they have attempted to email me at something@weebbiz.com or something@weeb.biz.co.uk or some other mangled variation of my domain.
James