go to network settings and manually connect to O2-UK network. that's the only option available while roaming on 48 in the UK unfortunately. Also you need to make sure you have LTE/3G/2G (automatic) switch bands on, it won't connect to LTE (4G) though. If it won't work, select 3G/2G (automatic) and make sure you choose the network manually. Switch off VoLTE option if you can see that, and also try to switch off SIM2 if it's on. I was in the UK last week and it was working ok while second sim off, only 3G or EDGE internet was working... worth to try
