I use the OpenRent AST agreements, and assuming the wording is the same as mine, the break clause can be activated after 4 months by giving at least 2 months notice.
Whilst they can leave the property whenever they wish, they remain liable for paying the rent for 2 months from the date they gave notice, whilst within the 12 month period of the AST.
“Mutual Break Clause
12.6. The Initial Term of this tenancy agreement may be terminated by either party giving the other at least two months’ notice in writing, such notice not to expire until at least 6 months after the start of the Term. A notice served by the Landlord under section 21 of the Housing Act 1988 shall be sufficient notice under this clause.
The Tenant is obliged to pay rent up to and including the termination date, so if the tenancy is terminated on a date which is not the last day of a rental period, the rent due for any incomplete rental periods will be apportioned accordingly.”
As a minimum of 2 months notice is required, the tenancy can end a bare 2 months after deemed service, but rent is due until 19 June. You are at liberty to agree something different if you wish.