Hi Paul - This is a big question so I will summarise my understanding.  Note, even where not required, the importance of the spirit of building regs as opposed to legal requirements requiring smoke and intumescent seals cannot be overstated as they do save lives. Intumescent seals are most commonly required in doors or door linings, but also in glazed transoms above doors, door viewing panels, where pipes run between rooms enabling the spread of fire and in flame retardant paint products.The seal when heated expands preventing flames and smoke filtering through from the areas affected by fire, to those areas designed to be protected, so as  to enable preservation of life on the other side of the door.  Initially prior to the spread of flames and burns, smoke seals provide some advanced protection from asphixiation caused by the spread of smoke and toxic fumes. Intumescent smoke seals ARE required in certain situations - but not just seals as you will see!: Some of these situations include:

1. Large HMOs i.e.. Mandatory Licensed properties on all bedroom and common-habitable-room doors (excluding: cupboards, bathrooms and toilets and entrance and exit doors.
2. In addition fire doors secured with a set of three fire hinges retained by fire door stops which are glued and screwed to the door liner, and finally fire rated door closers are also required to complete the required fire rating.
3. Half hour rating is normally required but in exceptional situations 1 hour rated doors are required e.g. two separate properties with a connecting door would be required to provide one hour for occupants to escape the fire - thus one hour fire rating.
4. Most houses do not require any of the above measures but it is our policy to fit them in any event - particularly following renovation as the additional cost is negligible.  It also future-proofs against subsequent further legislation requiring yet higher standards of protection.
5. Where the door finish requires painting it is prudent to use intumescent paint as opposed to conventional wood-painted finishes.
6. Non-licensed small HMOs may not require all doors to be fire-rated but some doors if exposed to additional risk might be required to be fire rated.  This would include a bedroom accessible via a kitchen, so that in the event a fire starts in the kitchen the adjacent occupant might not perish before escaping from an alternative exit such as a window..  The fire door would give the occupant additional time to escape via the window.  The window opening should allow a width of at least 450mm to enable the largest occupants to escape.  If the window hinges obstruct an escape or reduce the space to escape, then egress hinges should also be fitted to that room window or windows.