They do not use an interbank rate. HSBC use the standard Mastercard or Visa rate so no less favourable than Monzo (“By selecting to pay in the local currency HSBC will do conversion using the relevant payment scheme exchange rate applied on the day the conversion is made.”) and unless you have a premium account they also charge a cash fee (“Cash Fee 2% (Minimum £1.75, maximum £5.00)”).
Barclays also do not use the interbank rates but the card scheme rates (Visa for most UK cards, Mastercard for some foreign cards and some corporate) and say “if you’re…using a cash machine other than a Barclays cash machine or a cash machine at a Global Alliance member bank, you’ll also be charged a £1.50 Non-Sterling Cash Fee”. They do specifically note that the exchange rate is not as per date of transaction but a few days later when transaction finalised. Barclays (on non premier accounts) also limit withdrawals abroad to £300 (“You can take out up to £300 a day”).
Sources: hsbc.co.uk and barclays.co.uk websites 16/12/2017 and barclays personal customer tariffs pdf file