For what it's worth, whatever one's personal beliefs, and whatever insurance might be in place, I'm pretty sure that I wouldn't be doing it as a volunteer. I'd say that changing lightbulbs in a "normal" room is one thing, but that changing them in a roof at nearly 7m is quite another, and as already said should be left to people used to the work day in day out.
There are raising and lowering systems for difficult to reach lighting. As part of the "risk assessment" then what about them fitting these so that the lights can be changed at ground level?
I'm like the OP, in that I used to work at height on occasion and was used to doing so. I've also changed any number of light bulbs in badly placed, dangerous to access positions. I wouldn't be doing it now unless I was being well paid, and had the correct gear and training to do it properly and safely.