In a 12-month AST is it possible for the tenant to have a break clause allowing them to terminate on one month’s notice within the first six months of the lease or would that mean that the lease would not be an AST?

Compatibility of break clauses with ASTs

Section 19A of the Housing Act 1988 (HA 1988) provides that any assured tenancy arising after commencement of section 96 of the Housing Act 1996 (HA 1996) (which was 23 August 1996) is an Assured Shorthold Tenancy (AST) unless it falls within a paragraph of HA 1988, Sch 2A. HA 1988, Sch 2A does not contain any minimum period for an AST. Nor is a tenancy required to have a minimum length to qualify as an assured tenancy under HA 1988, s 1. There is therefore no doubt that a standard residential tenancy granted today on terms that the tenant is to have a right to break the tenancy within the first six months will be an AST.

You may have had in mind the pre-1996 regime of opt-in