do I have to apply for planning permission or multiple occupancy or anything of that sort?
Some L.A require planning permission to convert garage door on front elevation to a window, but most L.A see conversion of garage to habital space as Exempted Development. Check with your L.A.
The sun room extension will be Exempted Development if constructed to the rear of the garage & isn't visible from the public road. See conditions of Exempted Development.
Technically planning permission is required to sub-divide your property. However, as the Granny flat is attached to your property, will accommodate a family member, won't be rented & won't be sold as a separate property, then IMO it doesn't require planning permission. Especially as you intend to reinstate the garage conversion as part of your home in the future.
Anything in particular I should be careful of in terms of the conversion? (I know garage isn't insulated - should it be dry-lined?)
Insulate the garage walls, roof & floor (if possible). Dry-line or externally insulate wall or pump walls if they are cavity wall construction.
It may be possible to externally insulate the garage walls - if its only 2.5 walls then its worth considering. SEI grants are available to upgrade wall & roof insulation in your home.
Garage is flat roofed, was thinking of putting a pitched roof/skylights in to improve the lighting - is that feasible?
Assuming that the existing garage is attached to the side of your property, on the front elevation then a new pitched roof will require planning permission. A lean-too or hipped pitched roof may be feasible - depending on your house type, i.e typical semi-d.