Proof of ID when buying foreign currency.

[So]

My children, being the frugal little darlings that they are, had an accumulation of old £10 notes. I also had some, as did Mrs So. We are going on holiday later in the year, so used them to buy some Euros at the Post office.

The total was just over £400, and I had to provide ID for AML purposes.

But what happens to the information provided? Does it get stored with a central agency, then if I buy currency with cash regularly I get a knock from the authorities?