How to get tiling on uneven walls

the very best advice would be to have a plasterer level the wall for you failing that batten the wall to a level finish.with 2 x 1 inch studwork i can complete this for you.

For the very best results I would take the wall/full bathroom back to brick., Dot & dab plasterboard wall(s) which would get you a perfect plumb & even surface if plasterboarded correctly. I would advise a professional tradesman like myself for this work.