As the UK is such a big financial centre, the UK banks are very, very careful about money laundering and fraud. They have all sort of semi-automatic systems that suspend bank accounts at the merest hint of fraud.

Here's what has set the alarm bells off:

Many domestic bank customers are transferring large sums from their bank accounts to the Bitcoin exchange's accounts. Most domestic customers do not really do any account to account transfers, so this is a big red flag.

Fraudsters gain access to peoples bank accounts with scams and trojans etc., they need to transfer the funds somewhere so they can then extract the money. Often they pay people to use their bank accounts to do this - students etc. But some of them are transferring to the Bitcoin exchange's accounts, converting to Bitcoins and then escaping with those. This is perfect for them.

Once the banks have detected that even just ONE of these scams have been attempted, they will shut down the target bank account and won't even discuss it.

The banks also have multiple suspect accounts that they are watching for money laundering etc., and when they all start transferring to the Bitcoin exchange's accounts, the red flag goes up.