Before the Tenant Fees Act came into force on June 1st, 2019, letting agents and landlords could charge tenants for almost anything: referencing fees, administration fees, renewal fees, check-out fees, and a long list of other charges that often added hundreds of pounds to the cost of moving into a new home. The Act banned all of these.
The only payments a landlord or agent can legally ask you to make are: rent, a security deposit (capped at five weeks' rent if annual rent is under £50,000, or six weeks' if above), a holding deposit (capped at one week's rent), payments for a change to the tenancy requested by you (capped at £50 unless the landlord can demonstrate higher costs), payments associated with early termination requested by you, and charges for late rent (only if rent is more than 14 days overdue, capped at 3% above the Bank of England base rate) or lost keys.
If a landlord or agent charges you a fee that isn't on this list, it's a prohibited payment. You can report it to your local council's trading standards team, and the landlord or agent can be fined up to £5,000 for a first offence and up to £30,000 for a repeat offence.