My next home project is to convert the attic. I was wondering about
insulation. Should I use loose stranded fibreglass insulation like
Rockwoll,
or a solid insulation?

Conversion of a loft comes under building control. There are many, many
issues that must be addressed to make it legal. You'll need to consider the
following (and loads more I've forgotten).

1. Insulation (for maximum thinness, this is best done with
Kingspan/Celotex. There are loads of rules about air gaps, cold bridging,
thickness, breathable membranes, vapour barriers etc.)

2. Fire escape (usually you need to make your stairway fire proof (no open
plan lounges) and install an escape window in the loft. There are many other
methods of complying.

3. Fire alarms. You'll need mains powerered linked smoke detectors.

4. Fire protection. You'll need fire resistance between the top floor and
the others.

5. Foundations. They will have to be strong enough for the extra weight.

6. Walls. They will have to be strong enough for the extra weight.

7. Floor joists. They will almost certainly need replacing or supplementing
with much stronger examples.

Christian.