Ask HN: Financial startup asks for private info via email. Where do I complain?

I've been using the services of TransferWise (http://transferwise.com) to send money to Europe from the US. Their service has work out really well so far, but I'm worried about their approach to obtaining additional information for their fraud detection/customer verification service.

Recently after initiating a transfer, I was asked via email for "... a screenshot from online banking, a bank statement or a check showing both names of the account holders" and "... a screenshot from your online banking or a payslip, we would need to see your bank account number, the date, amount and TransferWise as the recipient...". All of this via email!

When I responded to their customer support with my concerns about the safety of supplying banking information such as statements and account numbers by email my concerns were ignored. I don't think that I need to explain to anyone here how dangerous it is to ask for this kind of information by email, and how this sets up expectations from your customers which could be easy exploited by Phishers.

So my question is, where is the best place to complain? It looks like they are registered in the UK: https://transferwise.com/terms-of-use-us Is it illegal for a financial institution to ask for their customer's financial information by email?