Roughly £8k for a rewire, £10k for a replumb (but should include a new CH system if you get a baxi boiler or some such). If it’s the same as our 30’s terraced house then the walls will be rendered with cement rather than bonding and shouldn’t need redoing, rather should simply require a skim over and patching where the plumbing has gone. Call that another £5k (for the whole house and ceilings).
So yeah, £20k will get you new wiring, plumbing and a fresh plaster job. But then you have a blank canvass to work with!
Things to watch out for- look for any issues around penetrating damp. Bad guttering/flashing can cause damp/cold walls which is an issue in winter (as the walls will be solid brick 9” thick walls). Also our genius previous owner had removed chimney breasts from the second floor but not the loft so we had to put a gallows bracket in.
Otherwise these homes tend to be good, solid, reliable homes. If you are going to replaster it is definitely worth having the insides of the external walls lined with 1” of insulation. It’s not going to make your heating bills drop to zero but it will prevent any cold walls in winter sucking up your heat and will make the house significantly more comfortable. Obviously more insulation is better but 1” isn’t much room to lose and if you strip back the render to brick you’ll lose less than 1”.
If the wiring or plumbing has been done recently then you may not need to redo the lot.
OH also- very important- get your builder to upgrade your water supply pipe. These houses were all built when 12mm lead pipe was the standard. You upgrade it to 25 or 32mm polypipe you won’t believe the difference it makes to your water pressure.
Also if you redo all of this, take the opportunity to get a hatch to your crawl space cut into the floor so you can get under there easily. We didn’t have one and it was a massive pain in the backside.