Highway Code for cyclists

I think you already know that where the HC states 'must' it is quoting or referring to legislation. Where it does not say 'must' then it simply advice proposed by the dft, commented on by interested bodies then published as informed advice, regardless if they took any of the comments onboard.

Dangerous v careless, ask the cps. It seems dangerous is a there be dragons issue when they can get a lighter but easier conviction using careless.

Not a lot. From the official Highway Code site:
Although failure to comply with the other rules of The Highway Code will not, in itself, cause a person to be prosecuted, The Highway Code may be used in evidence in any court proceedings under the Traffic Acts (see The road user and the law) to establish liability.

So basically if you don't end up in court there's no reason to follow any of it.

A number of judges have directed jurors to disregard the code when reaching verdicts on the grounds that it is not law. One example here:
http://beyondthekerb.org.uk/2014/02/12/futility/

In this example the judge directed the jury to ignore the advice on rules 93 and 237, advising drivers to slow down or stop if dazzled. Because why should motorists slow down if they can't see where they're going? Clearly the dead person and the person who got airlifted to hospital are necessary sacrifices to our need to get everywhere quickly even when we can't see the fuck where we're going.

(Pardon my French but this makes me so angry).

I have definitely heard of others, including more recently, but can't find the info now.

IANAL but I wonder if there is a possibility that the judges telling jurors to disregard the Code is precedent forming. I guess it doesn't matter given that first paragraph.