I'm Just about to go through a similar exercise. I have a chippy mate
who's going to make me a false wall. It will be made of 2 x 2 for the
battens/ framework and the side bits/front will be made of 6mm plywood.
My wall is to hide some ugly pipework and to also provide a backing for
a back to wall toilet the flush (no pun intended) type. a semi pedastel
sink will also be hung on it hence the 2x2 which should give a nice
strong frame.

I would say that a plumber probably wouldn't entertain the building of
said wall but a chippy would knock you one up in no time (if you can
get one) or DIY it, which again is simple. I just dont have the time
myself due to all the other jobs I have on. Make sure you know where
all your pipes will go though before embarking on this!

HTH

Cheers

Richard