I would supply from the lighting supply, however my loft is boarded, so I can safely exit the loft with a power failure. Of course same applies to rest of house, I did have two rechargeable torches which would auto light with a power cut, one has failed and needs replacing, I have a high socket on the landing to power the torch.
However should the lighting circuit only fail, the emergency torch would not auto light. So down to a risk assessment. Even your mobile phone is often enough light, and I have been caught out walking from car to house, when street lights turned off, I don't know what time they turn off, but even if I did, one does not tend to check ones watch to see if they are likely to turn off.
So in the interests of safety, it would be better if my plug in torch was powered from the lights, however likely my wife would use socket for vacuum cleaner, maybe should call it an air volatility cleaner, or may get picked up for using wrong word, as it can't produce a vacuum.
It is your house, so you must do the risk assessment. Can't see my wife lugging a vacuum cleaner into the loft.