Preemptive arrests for planning a protest seems like something that would spark uprising, I don't see it happening. Preemptive arrests for someone plotting violence? That might happen. Either way it would be wildly controversial.There are even noises that the authorities may “adopt the controversial tact of making pre-emptive arrests.” This news, among other items, has fostered fear among activists seeking to discuss and organise protests, without a doubt, using social media.
I would be curious to know what "noises" means. It's not that I don't think that the british government is capable of such things, just that it's a pretty vague reference.
I can't speak to the UK, but in the US... it is pretty close at times it seems there is a thin line between "conspiracy to riot" and "conspiracy to protest"