In any building, a ceiling helps slow the spread of fire. The ceiling will have a fire rating depending on how well it slows the spread of fire. If you install a recessed down light into a ceiling, chances are you are reducing that fire rating. The hole you cut for the fitting lets flames spread between floors more easily, often causing the ceiling to collapse within a few minutes instead of the usual half hour or more.
Fire rated lighting includes special materials that expand when heated and seal the cut-out. The aim is to slow the fire long enough for anyone overhead to escape. Fittings such as our Dennit and Daviik have an intumescent layer (https://www.youtube.com/watch?v=_Rh4C3FpJ8o) that expands when it reaches a certain temperature.
Most fittings are rated at 30, 60 or 90 minutes, each of which has a different purpose and place. The fire rating of a down light must match the fire rating of the ceiling, maintaining its minimum period of fire resistance. 3-storey houses have a minimum 30 minute fire-rated ceiling requirement, whilst a higher four-storey house typically has a 60-minute rating. A taller residential building whose top floor stands between 18m and 30m high will require 90-minute ceiling protection, whilst anything above that needs 120 minutes.
In a standard two storey dwelling, fire rated down lights are not a requirement, but they are highly recommended. When a property rises above three storeys (even for a simple loft conversion in a two-storey house), the Building Regulations change quite noticeably with regard to fire rated lighting.
If you don’t want to change your existing down lights, you can always have fire hoods fitted but the cost of the hoods plus the cost of the fiddly installation would probably work out the same, if not more expensive than swapping your fittings.