The LAW (as @meg54 puts it) for those who are still arguing that a landlord doesn't have the right to end a fixed term contact early to return to live in their former home. A landlord does have the right to do that. Standard ASTs will usually have a clause asserting that right. Unfortunately for OP's landlord, she either got poor advice or downloaded a shonky free online AST that didn't have that clause.
From the Housing Act 1988:
SCHEDULE 2
Grounds for Possession of Dwelling-houses let on Assured Tenancies
Part I
Grounds on which Court must order possession
Ground 1
Not later than the beginning of the tenancy the landlord gave notice in writing to the tenant that possession might be recovered on this ground or the court is of the opinion that it is just and equitable to dispense with the requirement of notice and (in either case)—
(a)at some time before the beginning of the tenancy, the landlord who is seeking possession or, in the case of joint landlords seeking possession, at least one of them occupied the dwelling-house as his only or principal home; or
(b)the landlord who is seeking possession or, in the case of joint landlords seeking possession, at least one of them requires the dwelling-house as [F1his, his spouse’s or his civil partner's] only or principal home and neither the landlord (or, in the case of joint landlords, any one of them) nor any other person who, as landlord, derived title under the landlord who gave the notice mentioned above acquired the reversion on the tenancy for money or money’s worth.
Or, as meg likes landlord zone so much:
www.landlordzone.co.uk/information/ground-one-repossession-of-own-residence/