Double brick = no need to insulate external walls as the cavity provides the insulating effect. Plus the wolf can't blow your house down.
My understanding of the sealing is that underneath the plaster is concrete which loves moisture and (as tinkster said) if you paint without sealing, as it dries out over the first 6 months it will suck moisture from the wall/paint and the paint will flake off. In theory it should be ok after that drying period without full sealing but I have always been told to seal a new wall regardless.