SuzySue wrote:

If a terraced house had a slate roof with no underfelt, would this be
something that most people would want fixed immediately i.e. taking
off all the slates, and installing underfelt and replacing all the
slates again.

Or is having no underfelt on a roof not a major problem, and something
which can be lived with i.e. not usually considered an "urgent"
problem.
It is certainly not unusual to get roofs like this, especially in the
North. The consensus seems to be that the lack of weatherproofness is
offset by the excellent ventilation (which quickly dries out any water
or snow that does blow in)! The roof space will also be pretty dirty.

I would say that if the roof looks in good condition (i.e. no broken or
displaced slates, no rotten or broken timbers) then this is not a major
problem. If you do need it doing, it is not normally hugely expensive
(maybe a couple of thousand pounds?).
tony