The sum insured for your surgery should represent the full value of its contents at today's prices. If this value is insufficient, your insurance provider is correct in saying this could affect the amount you receive in the event of a claim.
This is because you are only insuring (and therefore paying the premium) for a percentage of the true value at risk. For example if the real value of a surgery's contents is £10,000 and they are insured for £7,500, this means you are only insuring for 75 per cent of the risk. Based on this, the insurance provider would only pay 75 per cent of any claim amount submitted.
The best way to insure for the right amount is to regularly check how much the contents are worth, paying attention to items purchased or disposed of. It is best to do this just before the policy renewal date each year.