There is not a set ceiling for how much rents can be increased, everything should depend on what is in the lease.
Does the lease have a rent review clause? Is that the every 2 years' by inflation? If so, then the rent increase should be calculated against inflation. There are many lists of inflation rates available online. For example:
U.K. Inflation Rate - Historical Data
YearInflation Rate (%)Annual Change
Otherwise, if the lease is silent on rent reviews, revaluation and negotiation in the open market is the most common method of recalculating a commercial rent at review date. There is not a set amount or ceiling as to how much a rent can be increased unless it is set out in the lease. It would be useful to find out similar rent prices for similar units nearby to get an idea of what is being charged.
If parties cannot agree a new rent, the rent review clause in the lease usually stipulates a procedure for a third party dispute resolution - you will need to check the lease to see if there is one. If there's no rent rent, the lease might not say anything about dispute resolution.
If you can't come to agreement over a new rent, then RICS runs a Small Business Scheme for rent reviews, which may be worth looking at.
I trust that this has been of help. Any queries, let me know.