Class actions wouldn't apply here - I'm no lawyer, but my understanding is that the act only allows a form of class action for cases of anti-competitive behaviour, not for compensation for a failure to provide services like this.
There is going to be some kind of compensation offer from O2 for consumers - they aren't legally obligated to do anything like they are for their government and large business contracts, but they'd be pretty stupid to do nothing considering the bad PR. But it's unlikely to be much, probably some percentage of the month's bill back (though, from what I gather if this were a class action in the US the customers also wouldn't stand to make much out of it). If you suffered larger actual losses than just being without service you can make a claim under the consumer rights act but that would have to be for verifiable material losses, and substantiated with evidence - For example if a plumber had to cancel a job they were hired to do because they couldn't get in contact, you can probably claim for the cost of that lost work. But if it is about potential losses, that's a lot dicier - For example the same plumber who had no scheduled work for the day but would expect to get a job in that time but can't prove they actually missed out on a job because there's no record of the call out they didn't receive. There may be other avenues for a claim, but I'm no lawyer so I can't comment on that, but unless there is a serious consequence as a result of the outage I don't think there's a huge scope for a useful lawsuit.
I imagine that anyone who has to upgrade in the next few months have an extra arrow in their quiver if they want to negotiate a good deal on an upgrade. And I don't envy anyone who has to take calls or work in stores - Christmas is always a busy period, it's only going to get harder when you add a lot of customers that are already upset or angry.
Disclaimer - I do work for O2, but I can't comment on anything for the company, I have to leave that to the PR people - these are just my own views.