The SC didn't rule that the obstruction of the highway offence didn't apply in cases of protest. The question is whether the exception in the Act applies:
“if a person, without lawful authority or excuse, in any way wilfully obstructs the free passage along a highway he is guilty of an offence” (Highways Act, 1980, s.137).
They ruled that, under some circumstances, 'proportionately' exercising the right to protest could qualify as a lawful excuse. In the case in point, the protesters had blocked one of multiple entrances to an arms fair, in circumstances when only arms fair traffic was likely to even notice the protest (it was an entryway to the Excel centre). Also, the protest was of short duration.
The consequence of the decision is that the question of lawfulness is now case-by-case, as it is with other highway obstructions. There is essentially no chance that protesters blocking motorways are going to escape a conviction on the basis of this decision. They're also likely to be given post-conviction orders making repetition imprisonable.
FWIW I pretty strongly support the campaign's expressed aims, but reckon that this is some of the shittest praxis I've seen from protest group. Like the previous stunt by a bunch of XR-aligned people of gluing themselves to public transport, it's got a tenuous link with the aim pursued and is going to cost their cause vastly more public support than it can hope to gain.
[see https://www.cps.gov.uk/legal-guidance/offences-during-protes...]