What is the requirements for becoming a small arms repairman?

Hi, many thanks for your question - i think the trade that you are thinking about is the Royal Electrical and Mechanical Engineers (REME) Armourer:
From pistols to artillery and helicopter chain guns – as an Armourer, you'll know all the Army's weapons inside out. You'll use your first-class training to repair, maintain and adapt them.
https://apply.army.mod.uk/roles/royal-electrical-and-mechanical-engineers - This will give you more information about the role and the educational requirements for it.
Good luck and hope this helps.