any wiring for new sockets. Any pipes for new or larger radiators. Repair or fill any holes or gaps in the ceiling, e.g. around pipes or cables. If you have downlighters (vom) fit smoke hoods with intumescent linings, or rip them out and plaster the holes.
Remove the skirting boards (with care) so they can be refitted on top of your new boards
stuff insulation especially well round the edges of the room, against the walls (there may be a joist running parallel to the wall but the should be a gap. Hoover out this gap with your builder's vac first). This is because draughts usually come through cracks in walls or around joists and blow up through the gap under the skirting.
Any visible holes or gaps in the brickwork, hoover out, spray with water, and inject expanding foam (the pink fire grade is worth it IMO). It will prevent draughts and muffle noise a bit, especially in party walls, and prevent passage of smoke or smells.
There are people who say it is not worth insulating under a first floor as there is no heat loss (apart from draughts). I say it may help, and will slightly reduce noise. There are heavy grade insulating batts at builders merchants that reduce noise more than loft rolls, because noise reduction is proportional to mass of the barrier. I recommend quilt treated with Ecose, which prevents it shedding irritant dust and fibres while you work. It is brown, not yellow, and made by Knauf though also sold under own-brands. It will say "Ecose" on the packaging.
If you have a Lath and Plaster ceiling, hoover it out well with your builders vac (not a domestic one) and ask a plasterer if it is going to fall down soon.
Electric cables can lie on top of insulation, or under it, but must not be surrounded.
Fit a floor safe in a corner away from the door.