It is disappointing that the brewery agent recommended the supply company it did. Your case is typical of some of the bad industry practices where frequently agents' commission influences where the business is placed.
Estimates are often nothing other than a wild guess where the sum is based on the wish to provide an attractive figure encouraging acceptance as opposed to offering something based on factual information. The agent has no real consumption data as you were new in the establishment and prior records were with the departing defaulting tenant.
If the agent told you it was a fixed price for three years, he was wrong. The rates per unit or kWh (kilowatt hour) would be fixed but those would be applied to the kWh consumed regardless of the estimate. If the agent was driven by commission, the chances are that he was not acting in your best interests but in chasing his commission. In such cases, it would be you who is paying high kWh rates to generate enough to pay his commission. Also you were probably tied into a three or four year contract.
Cancelling a direct debit does not really help your case because the supplier will probably add 7.5% as a penalty and will continue to chase you for payment and threaten to take the legal route, thereby adding further cost.
Moving to another pub might not be an option but would automatically bring the contract to an end. You would then be much wiser in your fifth pub.
You could question the basis of the agent's decision to recommend the supplier he did. I do these things many times each month and there are ways of gaining release from the contract, but be careful - there are no guarantees and be sure you don't get ripped off there as well.