To vault or not to vault ?

[Rosecottageglos]

Hi considering to take down bedroom ceiling, which is the only up stairs room and vault ceiling . But…. Bitumen felt, but looks like a new roof has been built over the existing very old roof. So there is a gap between the old silver oak(?) and new rafters … surveyor said stone tiles externally to roof are fine just a few slipped etc. how to insulate whilst allowing to breathe ? Is it worth it ? Pic for context . Thanks, all , please no techie words as I won’t understand …