What exactly was their mistake(s)?

I'm not sure that "responsible/irresponsible" is a nice clean cut boolean as you're making out.

What was the great non-bailout from which the current credibility of the British regulatory system stems?

My answer is influenced by the facts that (a) the US is experiencing the subprime shakeout and (b) there's a housing bubble; it's a potentially unstable situation and people are jumpy.