Sorry to burst your bubble, but Lee Enfield and Brian dick have pretty much nailed it.
Where are Cantom & Wheaty?
First off Longbranch would not have used a Mk 1 receiver to assemble anything after the war.
You do not even have the bolt catch that would be used with the Mk 1 and the stock was not relieved to accept one.
The EAL furniture barrel sights etc. came from a military EAL.
This stuff did not even show up till around serial # 5000 so would not be found on an early rifle.
There is no way this is an EAL or a LB prototype.