Can we "trust" the Post Office?

As i was doing some research online about different travel products, I was amazed to get this screen from the Post Office. In summary their security certificate for the site is not correct and so up pops this massive warning from the browser (Firefox in my case).

It's so important to get the "trust" factor right with anything related to finance or online payments. It really doesn't help if something like this happens - especially when it's so easy to sort out.

The "untrusted" security certificate on Post Office website is a massive howler for brand reputation.