My original comment was that going to stock Android (Sony) would help identify if your issue is the GSI build or instead the device itself not being allowed on your carrier.
BUT, I didn’t cross-check the Unofficial Builds For Sony Xperia Devices list first: with the Xperia 1 ii on that list, I suggest you flash that, it will give better hardware compatibility than the GSI image.
Also, the builder of those images, @petefoth, is in the UK as well so could confirm the usability on the 3 network.