The frozen accounts could be that the constant activity triggers some kind of money laundering flag which the banks then investigate. Anyway it seems to be a right PITA to get coins in UK. I'm off to learn more about OTC. Will be brand new to IRC!
I think it is definitely a case of people using their personal accounts for trading. Banks would flag a huge increase in transactions as fraud, my bank has been frozen just for making some large purchases before.
Same deal with credit cards: it's happened to me twice in the past month. (The first time made some sense but I've no clue why they froze it again subsequently. Anyhow, that's been fixed, or so they tell me.)
Having no account to be frozen on a whim is one major benefit to using Bitcoin, of course.