I'm guessing the reason there is no lock is to allow meter readers to access it while you're out.
If you insist on locking it then you could probably replace that handle with a locking upvc door handle.
you might consider fitting some kind of window in the door so the meter reader can do his job
I believe the utility co's have an anti-vandal lock they fit to cabs in high-risk areas. You could ask for one. Does your have a budget lock operated by a triangular key?
if you can get a budget lock to fit, operated with a normal triangular key (the meter readers carry one) you will have brought it up to normal standard.
I doubt it would cost as much as £5
search for "meter box lock" or "meter cupboard lock"
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