Michael,
I wouldn't like to speak for Ian, so it's quite conceivable that I'm wrong, but...
AFAIK, once the next two aircraft have arrived, the fleet as a whole will be split between Denham, Booker and Blackbushe. There are local operating restrictions at Denham (caused by a combination of the owner, and being beneath the London TMA) which are more restrictive than IFR minima, meaning operating from there in less-that-ideal conditions is tricky.
There is a page somewhere on Ian's site which lists the interest received from different places. If I recall rightly, it was quite tricky to find and slightly out of date. My experience is the Ian is very open and approachable, and if there was sufficient interest (i.e. sufficient investors to cover a large percentage of the purchase price of a new aircraft) then new aircraft could be based pretty much anywhere.
Having said that, one of the benefits of the scheme is that you invest in a particular aircraft, but have access to the whole fleet. If you have a one-off aircraft up north somewhere, accessing the rest of the fleet might be tricky.
Rgds,
TPK