Was the credit and potential debit the same bank details? As in did you send yourself money and sending it back to yourself?
Did you try and send all of your account balance in one go as a new customer with new external transfer details?
Look at it from Halifax point of view - this ‘could’ appear as though you’re laundering money.
I appreciate exactly what you have done to gain £100 but don’t think you are going to find anyway around it at the moment.
In terms of the complaint - the bank should have taken it over the phone - this is number 1 rule and many advisors may ‘fob’ you off but they do need to register the dissatisfaction regardless of internal procedures.
Generally a complaint can be registered by any communication medium an organisation uses.
I know Halifax have very short branch hours at the moment so that will compound potentially the ability to go into branch.