You sound like you'd like the benefits of being a tenant without paying the rent that a tenant pays.
As a lodger, your landlord is entitled to bet here whenever they want. It is their home. It doesn't matter if they choose to be there all the days of the month or rent one or multiple other places for themselves. It is still their home and they are entitled to be there.
As a lodger, you pay a significantly reduced rent than if you were a tenant. A tenant rents the whole property or rents it with someone else from a landlord. The tenants then pay for the utilities and council tax and the property is their home and landlord needs to seek permission and give notice to enter the home. They will not be sleeping I the property.
You cannot expect to pay lodger rates and gain tenant benefits.
Currently, you've been on a very good deal because you have much freer access to the home without the landlord being present most of the time. However, you have not become a tenant because the landlord is rarely there. The notice terms for your lodger agreement remain whatever they were when you signed. As a lodger you are living in someone else's house, who is also resident there and who is the owner. You will have use of the kitchen etc but it is not your kitchen and so key decisions about its use etc will be made by the property owner, not you, as would be the case if you were a tenant renting the whole property.
You definitely sound like you're looking to have all the benefits of a tenant and have fuller control of the house than the lodger rent you pay entitles you to. Wanting to out a lock on the door sounds like you are trying to get one over on the Landlord in some way.
If you want a room with a lock, look for a room in an HMO. You will then find you are not living with the landlord but with other room renters with shared facilities such as kitchens. Your rooms are your personal space. I suspect you wouldn't find this alternative as appealing as what you have now, especially given that the majority of the time you have the house to yourselves.
I'd appreciate that you're probably onto a very good deal and not be looking to turn it even further to your advantage, in what sounds like a rather odd and unsavoury way.