o2 Issues Resolved After Over 24 Hours Of Downtime
o2 have have said that the issues with it’s network are now resolved. As we reported yesterday, this issue started early on 6th December 2018 and has taken over 24 hours to resolve.
Our 4G network was restored earlier this morning. Our technical teams will continue to monitor service performance closely and we're starting the full review to understand what happened. We are really sorry for the issues yesterday.
— O2 in the UK (@O2) 7 December 2018
According to reports in The Guardian o2 are working on plans to offer customers some form of goodwill gesture, such as free data or minutes, although the exact nature of what will be offered has not yet been finalised.
Alongside o2’s 25 million customers the outage also affected around 7 million customers of services that use o2 including Tesco Mobile, Sky Mobile and Lycamobile. It will be up to those individual providers to decide if they will make any kind of similar offer to their customers.

I am one of the editors here at www.systemtek.co.uk I am a UK based technology professional, with an interest in computer security and telecoms.