Rent Increase clause

Hi everyone, is it not legal to add a rent increase clause in the 12 month short hold agreement? I am only trying to add a clause which says that at the end of 12 months, rent will increase by the average of previous 3 months RPI inflation rate at a minimum.

Most tenancy templates have removed rent review clauses as its not in a landlords interest to tie their hands to a specific figure. They dont know where the market will be in 12 months time, so better to just serve a s13 rent increase notice nearer the time based on up to date data.

There have been a number of posts on this and other forums in recent months from landlords who’s tenancy agreement says they can only raise the rent by 3% or 4% when rents locally have gone up by much larger percentages and their mortgage costs have increased 4 fold.