Depends on the bank used in the UK and the bank used to withdraw the money from

There are banks ( such as Nationwide ) which have no atm fees and no currency commission charges and banks ( such as Barclays ) which do not charge a per transaction fee if using a partnet bank

The fixed fee otherwise would typically be 1.5% with GBP1.50 min

As far as buying some food, my recollection is that most outlets will take cards for payment , so just pay on debit card and there would be no fixed fee nor ending up with more HKD than wanted