Electric Heatpumps.

Um, yes. Plus, on the Victorian terrace, external cladding (and its eaves extension) would have to follow round the bay window (which has a tiny flat roof), and account for the elaborate mouldings on the windows (arched tops), doorway, windowsills, and, gratuitously, marking the first floor level. I painted it in 2019, there's barely a half square metre that's flat. And cladding one of a terrace would be hideous.

Just not viable to clad the front, more doable at rear, but as you say, roof extension needed on both eave & gable. 

I guess the exercise would cost north of £20K. Maybe 40 year payback in financial terms. Probably a similarly tenuous payback in emissions. Not exactly a low hanging fruit.

More realistic, perhaps, to insulate internally, piecemeal, as rooms are redecorated.

Actually, internally, the Victorian builders put spaced lath & plaster liners, inside the solid brick walls. It seems to be unusual, the EPC box-ticking exercise hasn't a box for it, and a cursory online search didn't show me any performance figures for it. It does feel significantly more comfy than simple solid brick & plaster. 

Um, yes. Plus, on the Victorian terrace, external cladding (and its eaves extension) would have to follow round the bay window (which has a tiny flat roof), and account for the elaborate mouldings on the windows (arched tops), doorway, windowsills, and, gratuitously, marking the first floor level. I painted it in 2019, there's barely a half square metre that's flat. And cladding one of a terrace would be hideous.

Just not viable to clad the front, more doable at rear, but as you say, roof extension needed on both eave & gable. 

I guess the exercise would cost north of £20K. Maybe 40 year payback in financial terms. Probably a similarly tenuous payback in emissions. Not exactly a low hanging fruit.

More realistic, perhaps, to insulate internally, piecemeal, as rooms are redecorated.

Actually, internally, the Victorian builders put spaced lath & plaster liners, inside the solid brick walls. It seems to be unusual, the EPC box-ticking exercise hasn't a box for it, and a cursory online search didn't show me any performance figures for it. It does feel significantly more comfy than simple solid brick & plaster.