As has been said your tenancy is almost certainly an assured shorthold tenancy and does not need to be renewed it continues month by month and there should be no cost involved if this was not in the original agreement, there is an advantage to the landlord/agent in them not having to seek a further tenant, taking up references, signing a new agreement, taking a deposit and still risking a poor tenant. But they can recover the property quite easily, often by the issue of a section 21 notice
(section 21 of the Housing act 1998 as amended) which is one of the mandatory grounds.
They would be required to give you 2 months’ notice to quit; there is no reason why you should have to start another shorthold tenancy. If you are a good reliable tenant who pays the rent and looks after the property the landlord/agent should not wish to lose you, talk to the Landlord or agent and inform them you wish to stay, they should be pleased.