For aircraft sheetmetal work, you do not need a licence at all. Getting it in EU or UK is not that simple, for big airplanes it takes 5 years minimum (if no previous experience is credited), costs quite something (including you need all type ratings on wich you want to sign-off your work) Plus, you need to "exercise privileges" of your licence at least 6 months in last 2 years, and doing only sheetmetal might not qualify with some more picky Quality Managers/ CAA inspectors.
Good aviation sheetmetal workers are pure gold, I always admired some guys at my place.
Regarding your question on electric hammers - I dunno. Pneumatic seems tradition everywhere - there must be a reason.