A Tenants rent can ONLY be increased during the fixed term if the Tenancy Agreement contains a clause allowing such (Rent from A to B is Y thereafter X or the rent will be increased in such circumstances - but these must be defined, clear and fair - not unreasonable nor contravene unfair terms).
Where a Landlord includes bills they should consider including a clause called “variable service charge” saying if bills increase by X these will increase by Y or rent increases by Y
If your Tenancy Agreement does not permit for a VALID rent increase in the fixed term then a Tenant can only have their rent increased at the end of the fixed term if :-
When a landlord has served notice on a tenant under Section 13 of the Housing Act 1988 which proposes a new rent increase and the tenant reserves the right to apply to the Rent Assessment Committee to decide upon what it considers the market rent should be. Any increase in the rent will then take effect from the date in the landlord's notice.
A tenant who believes that an increase in the rent would cause them undue hardship can attempt to persuade the Committee to rule that this is so and arrange a later date for its decision.
Auctions are becoming the norm for BTL investment