Hi,
In this scenario, as there is a contract in place with the landlord for you to sub-let the room out, rent-a-room may still apply if you meet the criteria found in the following guidance:
Rent a room in your home.
In terms of if any tax may be due on such income, this would depend on the level of income and if it breached the £7,500 exemption, or collectively above the personal allowance.
Thankyou.