Owning your own home

Given how quickly you say mould forms it sounds like something is fundamentally wrong with the bathroom install. Out of curiosity does the mould appear low down then creep up the walls, or does it start high up and come down? Is there a window in there or is it a typical afterthought pit of despair type en-suite with no natural light?

Whatever you choose to do you are looking at a medium sized job:

Keep as bathroom

1. Remove (and re-use?) bahroom suite
   
2. Tear out and re-do walls, consider backing all tiles with wediboard or similar
   
3. Fix any plumbing boo-boos and install an efficient in-line extractor that is controlled by a hydrostat
   
4. Re-instate bathroom re-tile and re-finish
   
   

Convert to utility

1. Tear out bathroom.
   
2. Pull up floor to cap and/or redirect waste to desired location
   
3. Tear out walls
   
4. Bring in electrics to room for sockets etc (I don't know if it would be possible to use the feed for the shower by lowering the rating of the fuse on the CU you'd have to ask @Airhead or @Nef )
   
5. replaster walls
   
6. fit units / worktop /sink
   
   

Either way you are looking at spending a decent chunk of change, without knowing the specifics of your property I'd say keeping the en-suite would be the cheaper option.

It does sound like you've made your mind up, but playing devils advocate moar en-suite bathrooms = moar value in the head of most estate agents.