It's illegal for a letting agency to charge prospective tenants a fee to register with the agency. It's not illegal to charge a fee, for finding the accommodation, once a contract has been agreed between the tenant and the landlord. (It's actually common practice, with typical fees of around 30% of one month's rent, plus VAT).
Many agents will charge a fee for finding you a suitable property, plus you'll probably also have to pay for the credit check and maybe an inventory too plus a 'checking in' fee. The agent also charges the landlord a fee for finding the tenant, drawing up the agreement, management etc etc.