Big things are- if you replanted the walls then all external walls do in 1 inch thermaline. It’s not going to make your house passive by any stretch, but it will get rid of cold walls, which makes a huge difference. Make sure the builder fills all gaps in the thermaline with expanding thermal foam to ensure that there are no cold spots on your walls.
Ensure that your underfloor is properly insulated and all gaps are closed up. You are ideally looking for a house that doesn’t exchange air easily through the floor or chimney- this will reduce your heating significantly and make the house more comfortable. It does however mean you have to be religious about airing the house out once/day to prevent damp, but opening the windows for 10 minutes will still be better than having cold wind in your house mid-winter.
Air con in your south facing rooms. The last two summers have been made far more bearable with A/C in the south facing rooms (the north facing are ok as they are).
On your shared wall have the builder put acoustic rock wool on the party wall with a double layer of acoustic gyprock. You lose around 2 inches of space (so consider if this is an issue) but it will help to dreaden sound transfer between properties.
In the loft (if you aren’t doing an extension) PIR in all the loft joists and rockwool in the ceiling joists.
Upgrade your water supply pipe to 25 or 32mm. While you are refurbishing redo your plumbing. It’s not a huge cost if the walls are already stripped and it makes a big difference to your water pressure. Ensure all pipes outside your thermal envelope are lagged (principally the pipes under the house). If the boiler is old change it for a new system boiler (we got our baxi boiler & cylinder for £1,500).
After that it’s conveniences- if the walls are stripped might as well re-wire, and if you re-wire get your sockets done, raised off the skirting, put USB sockets in, redo your fire alarms so they’re all wired in (comply with the new regs and also means you never have to change fire alarm batteries anymore).
I’m sure I’ll think of more, but they’re the biggies.