What sort of material is this and how do I go about redecorating?

[portuguesejohn]

I'd certainly prefer to take down the bathroom ceiling, insulate between the joists, put up (probably an insulated) plasterboard, skim and redecorate.

I think it will be cheaper than getting the roof opened up (unless you have to have the roof stripped for some other reason).

It also has the advantage of replacing the old ceiling whose condition I would guess to be poor.

This is sadly impossible for me at this point, especially with everyone locked in the house, bathroom needs to be up and running!

I did manage to access the area in the loft and found no insulation in this corner where the shower sits! I've moved the discussion to the appropriate forum, maybe you could advise? https://www.diynot.com/diy/threads/how-to-insulate-behind-sloping-ceiling.541909/