I am getting a spinning wheel by the 4G symbol at the top of my screen quite often. It appears that my phone is not receiving data from the network in this time. I’ve ported over from Virgin.
Have you changed your APN to O2's, assuming O2 have have provisioned your account correctly for an iPhone? You may need to contact O2 on 202 for Contract, 4445 for PAYG, or contact O2 via social media, links below this post, @Williadj.