I have some LV halogen lamps I want to put in my airing cupboard, but
they are supplied with a plug-in type transformer.

I assume you mean the transformer is a wall wart type with integral plug?

I would use an unswitched 13A socket indelibly marked "lighting only" fed
from an adjacent 3A FCU on the lighting circuit. It probably isn't quite
compliant with regs, but it'll be reasonably safe.

The main safety issue is that when the next occupier plugs the vacuum
cleaner or a fan heater into it and the lighting MCB blows before the fuse,
it will cause the lights to go out and granny might fall down the stairs in
the dark.

Basically, to do it properly, you need to find a proper socket circuit
somewhere to tap into.

Christian.