I’ve converted hundreds of lofts. Every loft conversion to building regulations.
So here’s my definitive answer.
If you cut through a rafter, if you fit a window, if you fit a stairs, if you insulate if it looks habitable. You will need building regs.
Structural implications, fire escape logistics and then ventilation implications.
Technically that magic number of 2m is relaxed by most building regulations services down to 1.9 m
This is also on the top step so by adding a landing you can sometimes get around low-roof lofts .
For all building regs care you can crawl around your loft as long as you have that 1.9m on the stairs.
Then there’s the ventilation issue. You need 50mm air gap above your insulation and must make sure there’s no conflict as condensation is going to eventually rot your rafters.
As for structure I’ve seen some really bad errors over my 27 years converting lofts, some easily fixed some meaning a full roof restructure.
Anyone telling you that you can do what you like but
I’ve also met people that have been legally made to undo every bit of work. ( usually, as soon as the window went in and the jealous neighbour phoned the council )
The only way you’ll be safe is to ring the council and ask them what you can do.