Banks have always closed accounts that have not operated for a specified time (basically it costs too much!). The funds are transferred to unclaimed balances but remains the account holders and if an interest bearing account, that has to be paid when the funds are claimed.
It should be a very simple operation and it looks like you are dealing with untrained staff.
If you are phoning the call centre, ask to be put through to the Branch Manager making it clear you have a serious complaint, and if he is not available and doesn't phone back your next communication will be with the Chairman/CEO.
Depending on the length of time since the account operated there may be a problem with I/D. Just before I retired the Banks had been ordered to ensure that I/D was up to date on all their customers and had been given a timescale. A copy of your passport verified by your overseas Bank may meet their requirements, if it a Bank with overseas branches you may be able to visit them with I/D.