That's not in the CPS definition though apart from for electronic systems.
Well the CPS does not define if there has to be intent to endanger lives or not, just that the act does in fact endanger lives.
So under the law as I read it, they could consider an act of forcing a motorway closed and endangering the lives of the public who are using it as a terrorist act, even if endangering said lives was not their "intent".
As said, ringing a bomb hoax in does not mean you have the intent on actually endangering lives, it could be for the reason as you said, to "cause disruption", however just like this it could be considered a terrorism act because it did in fact endanger lives and did so to "advance a political, religious, racial or ideological cause".
For me, this group know 100% that they are endangering lives, I mean they'd ALL need to be absolutely brain dead not to know that, but they have individually and collectively decided that the amount they are endangering, or the amount of people who may or may not be hurt due to their actions, is not enough to put them off their act.