No service in the UK, what baseband modem firmware should I try and flash?

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](https://www.youtube.com/watch?v=tWdgWzFdla0) 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](http://www.sammobile.com/firmwares/database/GT-I9300/VDI/) . 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.