IF I've understood you correctly you're fitting a new bath?
What I would do is take out the bath, cut a whole section of plasterboard away [say something like a 1 ft square] so you're exposing the airing cupboard - you may find a stud wall in the way somewhere but it doesn't really matter]....then you've got plenty of access to redo the pipework. But you don't really need to chop the pipe work around that much....just cut the elbows off, clean up the pipework to take a flexible tap connector and you should ok. A 300mm tap connector will be too tight, I'd suggest a 500mm flex tap connector and then loop it round - should be fine you won't get a restriction. Then when you've finished, you can cut some thin ply of something to go over the enlarged hole you've made - in fact measure and cut this before you've put the bath in, much easier, so you can easily offer it up when the bath has been fitted. You need to get rid of the rigidity of the pipe work.
Hope that makes sense.
edit: looking at that again.....you might be able to do something without bashing the PB about....you might even be able to cut the existing 22mm copper pipe on the vertical say an inch after the elbow mand fir a 500mm flexible bath tap by, again, looping it round - they are quite forgiving. Think I'd try that first.
