There's a widespread misunderstanding that only the exterior is listed. In fact, the interior and exterior are both listed.
As AnonymousBird says, the listed building people are generally very helpful - but never forget that their priority is the building, not your lifestyle. They don't give two hoots if your family needs a bigger bathroom. Their responsibility is the building.
Listed buildings have the concept of 'repairs' and 'alterations'.
Repairs are officially fixing something damaged using the same materials that are presently there - so if you replace a knackered floorboard with a similar floorboard, that's a repair.
Alterations are changing what is there. So adding a bathroom is an alteration. Changing a bathroom is an alteration. These need Listed Building Consent, which is like planning permission, but for listed buildings. You have to put in an application and wait two months for permission, which may or may not be granted.
Just updating an existing kitchen or a bathroom doesn't need LBC
Technically, if you repair something with different materials, even if the materials are the original ones that should be there, that is an alteration. So we are going to replace the cement render on the outside of our house with lime render. This is entirely correct, and we all know it won't be refused, but we must apply.
You can do your own applications for listed building consent, or get an architect to do it. If you've got the money, I suggest getting an architect to do the applications, as it's a real hassle.
There is a point at which the conservation officer may stare at you meaningfully and say 'that's a repair, okay? A repair'. Do not stand there saying 'no, I think it's an alteration, so we should be putting in for listed building consent'. They are trying to save you and them a lot of work.
I should say that buying a listed building in bad repair is astonishingly expensive. We had not one but two surveys by specialist surveyors, and there were still a lot of problems they didn't find. You really shouldn't do it without having a large contingency fund (£25k at least?). If you want to discuss finances, please send me a PM and I'll talk about our budget so far. You MUST get competent, specialist people (we had the sodding electrician who ruined our plaster, for example) who are experienced in working with old buildings, and they must use the right materials.
My sister is a CO and her advice (which I often post here) is that you should never buy a listed building without being happy to live there without making any changes to the layout.
HTH