First off you need to know your predicted load, but I'd start with 6mm² as a minimum and work form there. You'll also need to size the outgoing fuse as well, an RCD is probably mandatory these days.
One thing to consider is that this work is part P work and is notifiable to the local authorities by law. You can notify yourself but the cost is quite high, if caught with no notification or certificate then the fine is/was....long time since I bashed cables....£5000
Personally, and I am an electrician, I'd get some one in, if only to give you rough pricing and options, some electricians are more than happy to let you dig the garden up and run the cable and they come along and do the connections and testing. Given the tightening on the law with electrical regulations I'd rather just pay someone these days for domestic work unless it's a simple socket change or spur.
Stupid really as my qualifications allow me to work on 600V + on machines capable of killing not one but several people in one go, but I can't put a new ceiling rose up or extend a ring main in a domestic property. Having said that, small works have been relaxed recently, but I'd need to get my notes out from the recent IEE course to be sure exactly what I can and cannot do without notification and/or part P certification.
and then I could build a building within the model buildings on the layout 
