Banks often have rules that make no sense and don't react to changes in customer behaviour.
I was once criticised for using my overdraft every month, despite having substantial savings with the same bank. I used my overdraft because it was interest free up to a certain amount so didn't cost me anything and I could have paid it off any time I wanted to, but didn't see the point.
I've never had problems withdrawing hundreds of pounds worth of foreign currency in cash but it was always paid back within a few days.
I suppose these days, when just about everywhere accepts credit cards, banks see the need to question why anyone needs large amounts of cash and might think that you've borrowed money from a friend, relative or unofficial source like a loan shark.
Plus, if you're only paying the minimum and are close to your limit, that probably indicates to them that you're in financial difficulty. You can remove the minimum payment marker by simply setting up a standing order to pay £1 over the minimum.
But if you are only paying the minimum or not much more, chances are that almost all the money you pay them is going on interest and only a tiny amount is going towards your debt.
There's now rules in place that stops this situation continuing for years on end - banks are required to send you down the 'persistent debt' route, which basically stops them from taking endless interest from you and keeping you in debt for decades.