fireproofing rain pipe

Presumably it's plastic? You can get an intumescent collar which goes round the pipe where it enters the ceiling. When it gets hot, it swells up, crushes the pipe, and seals the hole in the ceiling (in theory) making it fireproof. A good builders merchant will have them.

Easiest is best, so pack rockwool around the pipe and box it in with plaster-board, two layers, and it should be enough to pass the fire prevention inspection.

Fire requirements, especially for residential property, needs pipe boxes to be filled with rockwool (glass fibre insulation material), or other inflammable material, to stop any air draught rising through the box work. This is easily achieved after you have placed the first section of your box in place. Wrap the rockwool around the pipe, making sure to reach right in behind to the back of the pipe. Just make sure the rockwool fills all the space tightly enough in the box work to prevent free flowing air passing through it when it's finished.

Your pipe is now protected against the transfer of fire and smoke, and should, if the worst happens, give you a lot more time to escape from the building. After it's done, you should feel safer sleeping in your bed at night. :-)

Good luck with it.