you'll need a plumber to check the existing pipes but you may be able to access inside the wall from an adjoining room (easier and cheaper to replace plasterboard than tiles) and be able to add a new shower with a reachable shower rose as well as a handheld but i hope you have a few spare tiles or can steal some from a less obvious place ...best of luck with the plumber but research suitable new showers with handheld and adding the plumber's time and repairs expect at least $2-$3 thousand but well worth it for a more comfortable bathroom

Would you consider two mixers, a lower one for the bath and a higher one for the shower so there is no confusion for visitors? Safety and ease of operation should be the highest priority when looking at mixers for shower/bath combinations. If you only want to have one mixer then make the height suitable for somebody having a shower. You don't want to be bending over to adjust the temperature when showering in a shower\bath. I usually design bath mixers and spouts centered on the wall of the long side of the bath so if you need extra hot water when you are having a long soak it is easy to reach. I have never heard of the requirement for a bath mixer to be at 700mm, although this could be a good height for a bath spout depending on the depth of your bath. Best of luck with your new bathroom, Dr Retro of Dr Retro House Calls

my suggestion is similar to Paul's but with a lower counter (built in seat and or towel storage?) under the lower window sill with a 1500 wide vanity and wall cabinet... you could also have the bath against the window and have a freestanding storage cabinet or chair at the other end the vanity and showers ;locations haven't been moved but reducing the entry door to standard 820 would allow for a standard 1500 wide shower and the gaps around the door in the bedroom could be repaired with a wider architraves to save needing to plaster?

Removing the flow restrictor wont help, as the knuckles in the modern shower head set-ups are too small and wont allow sufficient volume of GF hot water through. Best detach the old rose head from the pipe/knuckle set-up, throw it away and then find a rose head that you can adapt to the old knuckle /pipe set-up. It's what I did when I reno'd my bathroom and every thing works fine - good flow with good temp. Plus, you can also buy pressure equalising washers for the cold tap which will give you even more control balancing the hot and cold water.