Use of home expenses Rent calculation

Hi,

Please note I am not an accountant.

I would kindly like to ask a fundamental question regarding calculation of use of a rented home as a sole trader.

An example scenario:

A flat is rented, it has 2 bedrooms plus a living-room that is connected to the kitchen without any doors, so it's like one room.

If 100% of one bedroom is used as a home office and nothing else.

Would it be reasonable to count the living-room/kitchen area as one room and thus for each rent divide the rent expense evenly by 3?

I.e. for example Rent divided by 3.

Or would you have to calculate the kitchen/living-room are as two separate rooms even though they are not separate.

I.e. Rent divided by 4.

The same question goes for heating/electricity bill, home insurance and council tax.

I would appreciate confirmation if you think this is reasonable and if not why.