Attic insulation between joists and rafters - bad idea?

Issue could be moisture between the roof and insulation, there is usually a membrane put in place to prevent that. However, the symptoms would be mould and moisture in the attic itself, not in other rooms which would be "sealed" off from the attic (at least to the level of a plaster skim) (and barring any breathing issues of the outer wall to the top plate).

Did you get a survey when buying the home? The surveyor should have remarked (good or bad) on the insulation. This should give you a good starting point

First check if there are vents in your soffit, if yes just pull back the section of insulation that's covering them. If your roof is tiled consider fitting 2 or 3 tile vent on both sides of house. It's just a matter of lifting out a tile and replacing with a tile vent.

I woukd not worry too much about it. Fiberglass/rickwool is breathable to an extent. They are using insulation to stick to ceilings of attics which I am bot in favour of as that insulation sticks to timber and will not allow timber to breath.

Looking at tge floor of your attic it looks like loose blown wool insulation. How many inches are there on the floor of the attic. It looks to be above the joists.Pulling the roof wool down might make it awkward to go accross the attic if there is an electric issue.

We have some ventilation/mould issues in a couple of the rooms that I am planning to address next year (proper ventilation of bathrooms, kitchen etc. - maybe PIV).

Pulling back the ventilation in the attic, at soffit should allow air in an ventilate the attic space. No harm in pulling say 2 sections of the sloped insultatio, on either side to reveal the membrane and inspect the timbers to satfisy yourself that no harm (sweating/rotting) is happening.

Also, if you have old style roof membrane you can also insert something in where the membrane overlaps to encourage air in - google "Roof Ventilation Lap vents"

But back to the issue, the ventilation in the house - for now just open windows daily and avoid very low temperatures in those rooms.

I retrofitted a partial MHRV in an old house - and it has been amazing. We never open windows and have a trickle of fresh air all year long. I only fitted to top (3 beds & 1 Bath) and middle (Kitchen, Bath, Office and Dining room) ; the ground floor sitting rooms are not included in the plan. Point I want to make is that although MHRV is best for air tight houses and passive homes etc. dont be put off partially running with it if you want to. The Humidity stats boost the fans and sort out the bathroom whilst the beds and living areas have air supplied 24x7 and move moist air ….hopefully extracted by the bathroom/kitchen due to air flow in the top two floors of house.

I appreciate you may not have access to under the floor - so good luck !