Owning your own home

that is very cool

Can't help with that but I grew up in Barton and remember that house. My best mate lived a few houses down from it.

Lots but would be worth it I expect

Electrics and flooring is the cheap bit.
The question you need to ask is how much is that going to cost to heat if you don’t somehow make it more thermally efficient?
All that glazing need to be brought up to date and there is lots of it!
A mix of internal and external insulation as you don’t want to mess with the fitted furniture would be on my list along with solar batteries and new heating (heat pump?)
280k if you were savvy, project managed and did some work yourself.
Shit loads more if there are 3 sign written vans with a considered typeface and proper kerning rocking up every day and an office manager who answers your phone calls about the shade of grey for the render.

I think Mr_Smyth has nailed it.

I think you'd also want to understand the impact and limitations of the Grade II listing in that area.

Can you put in whatever glazing you want? Or is it all going to have to be some pricey custom stuff with annoying parameters that mean you're barely any better off?

gently delineating zones for playing, relaxing and entertaining.

Man I hate this sentence. Fucking rich people playing in their geodesic domes

If it wasn't grade II listed it might be a reasonable job but that could easily end up costing up 400k as I think internals are considered as part of that status