How can I insulate this extension?

[MrsMogginsMinge]

We have a small outrigger extension (date unknown) coming off the back of our kitchen. The house is a 1890s-ish terrace with a raised ground floor. The extension has wooden floorboards with (I assume) some sort of joists underneath. The extension has a storage void underneath it which is accessed from the steps down to the rear cellar door from the garden. The extension bit of the kitchen is BLOODY FREEZING in the winter, especially when the wind blows towards it. You can feel the wind whistling between your toes from under the sink. I am determined to get this sorted before this winter.

Looking up from the storage void, it seems as if the previous owners have just nailed up some ply from the underneath. God knows what is behind that. There are some waste water pipes coming through the extension floor to a rear drain and I suspect the hole for those may be larger than required.

Can we somehow add some insulation from the underneath and/or plug up the gaps? Is this going to invoke building regs somehow? If I need a professional (as I suspect I do) what kind of person do I need, and how do I find them? I'm a bit wary of those companies flogging loft insulation etc.

Help!

On reflection I think @PigletJohn might be the man I need here

pull off the ply "ceiling" and clean away any rubbish. You can put mineral wool quilt (as used in lofts) between the joists. It can be temporarily held up with netting to prevent it falling out, then put up a new ceiling of plasterboard (which is fire resistant) and preferably have it skimmed by a plasterer. You can get the plasterer to fit the board if you like, he will probably do it better and quicker than you.

Use pink (fire grade) expanding foam to seal any gaps, especially round the edges or where there are holes for pipes. Mark the position of pipes and cables on the ceiling (1) to deter you from putting drills and nails through them, and (2) for ease of future maintenance.

Stuff the mineral wool especially thoroughly round the edges of the room, where draughts usually get in.

Attend to any plumbing or cable repairs or improvements while it is exposed.

The void under the floor must be well-ventilated to prevent damp. Lag any exposed pipes.

Amazing. Thanks PJ - you are a treasure.