A UK bank has frozen my personal and business accounts and is not honouring any DD or SO payments. They did this because my wife informed them two weeks ago that we had split up. Is this legal without a court order of some kind?

Nope, but what makes you think a court order wasn't obtained. Go to the bank and demand to see any court order freezing your accounts. If they can't produce one, tell them to unfreeze the account or face a lawsuit.