Strip out everything, removal of ware done by plumber. Tile removal you can do your self. You may need to re-sheet partation walls as chunks of them will come away with tiles. If they are block walls there will be no problem. you could also tile over tiles, tiler will advice.
Plumber fits bath and shower tray before tiling and adjusts any pipe work, e.g new towel rail etc. Tiler tiles and then plumber returns to fit toilet,WHB, shower, and shower doors and radiator/towel warmer.