Why a max of Â£500. May be a Halifax thing but I can go much more. So your "Maximum" is misleading.
You can get more then Â£500 out of an ATM can you?
Which bank accounts is this?
Of all the accounts I hold the maximum ATM withdrawal is between Â£300 - Â£500 depending on what account it is, but I am happy to be corrected, as I don't hold an account with all banks in the UK especially the Scottish banks, and make the assumption that they have a maximum limit of up to Â£500.
The withdrawal amount is around Â£300, unless it is increased by individual banks periodically, also the OP provides a range between Â£300 - Â£500.
That said my answer to the OP still stands regardless of the amount, if you exceed it you are given a warning on the ATM and told to contact your bank, where most of the time you have access to more money over the counter.