You can do lots of things on a listed building without consent.
However you have fundamentally misunderstood what Listing means. The entire building is listed, inside and out.
Good luck trying to speak to the conservation officer - in the good old days we could do this. Now with council cuts we can't. They will only speak to if have already put an application in, meaning you have to have a huge amount of knowledge already.
You can do generally upkeep and repair - ensuring that you use appropriate materials of course. You can't replaster a lime plaster wall with cement plaster.
However you could replace the boiler assuming you aren't going to go round making holes in the walls. Same with rejigging your bathroom layout.
Spotlights in the ceiling - not if the ceiling is a historic ceiling. Similarly levelling the floor - this would prob be seen as part of the character of the building. And what is your solid floor made of? If you wanted to level it you would need to use appropriate materials.
I've owned my Grade 2 building for 10 years and so far needed Listed Building Consent once and Conservation Area consent once (yes, I have the double whammy) but still done a lot of work. But owning the building is a labour of love, everything takes time and research, finding contractors who are listed building experienced but not extortionate is important. It will cost you more than buying a non-listed property.
I still love my house though.