Your in the UK why not just buy a deact gun ?? you guys cant own live guns there so building one would most likely land ya in the slammer .....
Right this is were it becomes hard to follow (said my local fire arms officer).
In the 2007 reduced crime act it stats that a reenactor can build or own a 1-1 scale replica top venting blank firer (i.e. big boys cap gun).
The vet has to be under 2 inch of the round chambering.
Also no original parts can fit on except mags.
(this has came from my local fire arms officer and people that have made them or in the proses of doing so.)