don't know where you live Susan but if an old property you will need to check if in a conseration area and if so may be rules on the type of window you can have- a friend of mine got in to trouble when he replaced some old sash windows as the new ones were not the right type
Whilst aluminium frames might seem like a good idea they are expensive if you want to avoid cold-bridging which happens with a solid aluminium frame. To avoid this the frame needs to be split (inner face and outer face with an insulator between) and this is expensive.
I'm also surpised that the planners say that you can replace sash frames with aluminium in a Conservation Area. You are being misinformed and maybe you need to check with the conservation section of (I'm presuming) Southwark Council. Which Conservation Area are you in?
The best option would be to replace the existing frames with hardwood versions. You don't need to use double glazing and double glazed units will make the glazing bars too thick. Single glazing with inner shutters and curtains do as good a job and look more in keeping.
They are not sash windows, but a wall of glazing which opens onto a seventh floor roof terrace, which can only be seen from the air. I have spoken to somebody at the council who knows my apartment well and he doesn't think there will be a problem. I will be having thermal break aluminium if allowed, if not I will have hardwood. A precendent has been set in the listed building nextdoor to me, and opposite me. Thanks, I'll let you know what I end up with.