Thanks guys.

Tac I'm not intent on making something which is illegal - very much the opposite.
I have no intention of pursuing the design if it's not permissible under Section 1.

The only relevant section I can see in the act is the section 5 prohibitions.
(ab)any self-loading or pump-action rifled gun other than one which is chambered for .22 rim-fire cartridges;
(aba)any firearm which either has a barrel less than 30 centimetres in length or is less than 60 centimetres in length overall, other than an air weapon, a muzzle-loading gun or a firearm designed as signalling apparatus;


(2A)In this Act “self-loading” and “pump-action” in relation to any weapon mean respectively that it is designed or adapted (otherwise than as mentioned in section 5(1)(a)) so that it is automatically re-loaded or that it is so designed or adapted that it is re–loaded by the manual operation of the fore–end or forestock of the weapon.

(9)Any reference in this section to a muzzle-loading gun is a reference to a gun which is designed to be loaded at the muzzle end of the barrel or chamber with a loose charge and a separate ball (or other missile)
This still leaves the definition of 're-loading' open to debate though so does little to settle the indexing debate.
I might still play about with a design and plastic mock-up but I think based on the points made it'll sadly not see production