Rent-to-rent is legal in the UK, but it must be structured correctly. Most Assured Shorthold Tenancy Agreements (ASTs) contain a clause prohibiting subletting without the landlord's express consent. Operating without that consent risks invalidating the agreement and the landlord's insurance. You also need the legal right to rent in the UK. If the lease explicitly permits subletting, the arrangement is fully legal and can be entered into as a formal rent-to-rent contract.