You are not talking about a room share. You are creating a self-contained unit. I am not on a rant, I am pointing out what might happen at the RTB if you try any smart alec method of claiming a licence. The RTB can be a very unforgiving place if you get it wrong. One bollocks of a tenant who will lie to beat the band and you will have a nightmare.
Someone with your fear of losing a case could just stick up a smart camera pointed at the door and you can have loads of evidence showing you entering and leaving the "self contained unit" thus proving maintained access and no exclusive use thus its no longer one word against another. Could also but a smart camera in the shared utility again proving shared usage, if you are so paranoid that is.