O2 have said they will refund customers for the loss of service last week. In a blog on their website they wrote:
As we said in our last update, we want to make it up to our customers for the loss of service some people experienced last week.
The issue we had was unprecedented and we recognise that this caused inconvenience and frustration to those impacted over that one-day period.
We have now identified all those customers directly affected (those whose devices could not connect on our system) and we and are giving them the equivalent of three days back for the disruption as a gesture of goodwill and to say sorry.