This is the governments site on apprenticeships, might shed some light or at least offer a contact to ask the question.

http://www.apprenticeships.org.uk/

This is not quite the same, but in America the NRA offer short training courses in all aspects of gunsmithing and stock work, take a look in the Training section on their website; also Montana Long Guns in the States offer a pretty well respected course in custom gunstock making

http://www.mtlongguns.com/gunfitting_academy.htm

Now this bit I might be totaly wrong with, but I'm sure I heard/read that Matthew at the BASC Firearms Dept was a former gunsmith/maker with one of the London gun trade, maybe one of the team at BASC may have an idea.

Follow your dream Alex; I intend to, Montana Long Guns here I come!