I have a Galaxy S3 running CyanogenMod 13 and I had always assumed it was locked or something because I could never get any service in the UK. When I went abroad I noticed I could get roaming service.
I came across this video which seems to describe my problem and a solution to it. I notice though that the baseband firmware download they suggest (
I9300XXUGMK6) is an Irish one according to Sammobile.com.
I9300XXUGMK6 is actually the baseband version my phone reports now. Is the problem maybe that I am running and Irish baseband version in the UK or does it not work like that?
EDIT: So I found out that
I9300XXUGMK6 is called
PDA and describes the device I have and there are a few different versions available for that device. The most logical baseband firmware for me then would be the I9300XXUGMK6_I9300O2UGNA1_O2U.zip since I am on GiffGaff which uses the O2 network. But I can't actually find this for download anywhere though.