Long term contractual rents, public or private are controlled by a national index. The legal contract will contain a clause that the rent can only be increased in accordance with that national index indices. It’s regionally based and rent levels vary.
The usual period for a contractual long term let agreement is 5 years but can be longer. Beware that even after that period evicting a tenant who has paid can be a difficult business. Also under the law the tenant does not pay either community charge or local council taxes so those costs need to be covered in the annual rent. Rental income has to be declared and quite honestly by the time you are finished from an owners perspective it’s worth very little. Finding a decent tenant who will pay and not trash the property is very difficult.
However in these investment times it’s probably the best of all solutions for those stuck with unwanted property.