Don't use squirty foam. it glues the tiles, timbers and plasters together so that repairs are nigh on impossible, and it tends to hold damp so you get rot.
If your roof is going to be repaired or retiled, that's a good opportunity to add insulation from above, while the space is exposed.
or if the plaster is cracked and sagging, pulling it down (a dirty job) and replacing is a good time to insulate from below. Lath and plaster is probably about a hundred years old and sooner or later will fall down.
Usually I prefer mineral wool, which is non flammable, but in your case I would go for foam slabs, cut and fitted very accurately, because it has about twice the insulating power of the same thickness of wool.
The least expensive option would be to get a plasterer to fix insulated plasterboard (which is bonded to foam slab) to the existing ceiling, and preferably skim finish it. Plastering a ceiling will cost some hundreds, it gets better value the more rooms you have done at one time. This board is not so thick so the insulation will be less, but you will notice the difference.