Have a word with your optician or even specsavers (who I used), they might even be ok with you taking the pistol/rifle in if asked and checking what prescription is needed, then its about £20-£35 for the lens to be made.

Specsavers allowed me to sight my pistol in the middle of their busy shop (a few funny looks from joe public), worked out what I needed and made the lens the same day £30 all in. Just make sure you get the anti reflective coating on the lens as you do need it to help keep everything clear.