Before "re-doing" our bathroom I was considering dry-lining the two
external walls with plasterboard, or even insulation-backed
plasterboard. Currently the walls get very cold at this time of year and
this of course is a great condensation producer.

I'm hoping dry-lining will help against this.

However, how far away from the physical brick wall would this put the
tiles? How do I then go about securely fixing things to the wall,
through the dry-lining?

Secondly, can I do anything similar for the ceiling? The ceiling itself
is also suffering from condensation soaking into it, cracking/peeling
the paint and the outer lining paper on the plasterboard, especially
near the external walls and all the area around the extractor fan.

Thanks