Rob Horton wrote:
My loft has a minimal amount of insulation that was probably put in
about 20 years ago. The house was built in 1984 and the loft is pitched,
unheated and ventilated.

Not sure of the correct terminology, but I refer to joists as the
horizontal wooden beams in the loft that the ceiling is fixed to and
what you actually walk around on. Rafters are the sloping wooden beams
that the sarking and tiles are fixed to.

(1)
I want to add more insulation. Current regulations say 270 mm for glass
wool, 250 mm for rock wool, or 220 mm for cellulose. Are there any other
rollable insulation systems that have a higher level of insulation thus
needing less height?

Not rollable. Celotex between the joists would be the alternative, but
more work, and a lot more money.

(2) If I need to add height to my joists, (it's a small house and need
to use the loft for storage) can I lay wood strips (of the same
dimensions as the existing joists) on top of the the existing joists?
How do you fix them in place, glue and the occasional nail or screw?

(3) I have electrical cables in the roof, although these are for
ordinary electrical lighting. Is it OK to have these covered by the
insulation? The cables are already covered with insulation by the
previous insulator/owner.

(4) Although it is an unheated and ventilated loft, will I get any
benefit from adding insulation between the rafters? I want to use some
thing that is enclosed in an aluminized bag so that it will (hopefully)
reflect some heat away during the summer.

Rockwool slabs between rafters works well if you use the loft enough to
make it worthwhile.
Nobody seems to have much faith in the aluminium/bubblewrap sandwich
stuff you see in the sheds

Thanks.