I thing that you could look at is building an external timber frame with something like kingspan in it, and leave a 50 mm gap between walls. The new outer wall could either be timber cladded or use brick slips if you want to keep a brickwork effect.
This would probably give you enough insulation, if not you could also go the cavity insultation way.
I have no idea if the building regs would object to this.
Just had another thought, if you are pushing the wall line out and you will definatly need an architect to check that the extended roof line would not interfere with the first floor window tops. It depends on pitch of roof ect.
The roof is fully supported and could just have extensions fixed to the rafters.
As far as the window reveals go, the windows would be removed and repositioned into the outer frame.
Let me know how you get on as I have never heard it done this way round before.
Might also be worth a word with the authorities as to if they have a prefered method of doing this kind of thing.
I know only that what is moral is you feel good after. What is immoral is what you feel bad after.