This page is only designed to give you an overview of the local requirements, but it is the host duty to know the legislation and to comply with the local laws. misterb&b is not a legal adviser : you should have a look at the relevant laws and contact the local authorities or a local professional for further understanding.
The 90-days exception
Sections 44 et 45 of the Deregulation Act 2015 has introduced an exception to the Greater London Council (General Powers) Act 1974 by allowing to rent freely for 90 days by calendar year. As long as a Host is subject to the Council Tax (it can be determined here), it is possible to let an accommodation for 90 days without applying for a planification permit.
Nonetheless, letting beyond these 90 days would be considered a change of use of the accommodation, that requires a planning permission from the Department of the Built Environment. You can find details about the change-of-use procedure on this page. Note that the procedure can be completed online on the Planning Portal.
Mind that the Local Planning Authority (LPA) of a borough can overturn this 90-days exception in some residential areas in order to protect “the amenities of the locality”.
The tenants can only sublet their space after obtaining some prior consents (landlord, condominiums) as required in their lease.
Safety and health standards
The rental units must meet the local safety, fire safety, maintenance and health standards.