In article ,
"Brian Reay" writes:

But, the foil will act as a vapour barrier, all be it an imperfect one,
which is a worthwhile reason to do it.

It's not.
The permiability of the plasterboard is very much lower than
the rate moisture escapes from even a poorly ventilated loft.
If the amount of moisture in the loft is an issue, then you
need to ventilate the loft, and it's much more likely to be
comming from a roof leak or header tanks, or holes punched in
the ceiling for recessed lamps, loft hatches, etc.
Forget about the tin foil.

--
Andrew Gabriel