Тут правда конкретно про eu роуминг, но возможно в UK, а может и по многим другим странам работает аналогично (т.е. на уровне "роуминговых" отношений/договорённостей операторов):

When the same network generation, for example 4G, is available in the country you are visiting, then you should normally have access to it. Your operator should ensure that you have access to the same network and quality as at home. In some cases, this will not be possible because the same network or technology may not be available in the country that you are visiting. Network availability varies across the EU, the latest mobile networks may not be deployed everywhere at the same time and the same mobile network speed may not always be available. This can be the case with 5G networks.

Т.е. вполне логично, что cablenet был только 4g