I have recently removed a load of grotty wall tiles from a small
bathroom. The walls are plasterboard on studding-even the external
walls.

Looks fine to me. I'd just peel off any loose paper and tile over. Use a top
quality adhesive suitable that claims suitability for swimming pools. Then
ensure that you follow the instructions, including, very specifically, how
to tile using a 100% contact technique. You do not want dot-n-dab for a
shower area. Possibly consider using a PVA sealer, but check compatibility
with your chosen adhesive first.

I'd recommend the more expensive BAL products for ready mix or Ardex
Arduflex for a powder mix. Don't touch anything that claims to be usable as
a grout as well. Don't touch anything that doesn't mention swimming pools.

Christian.