I don't know - so please, please take this with a massive grain of salt. My guess: export restrictions and/or supply.

You cannot support a foreign military without approval from a lot oversight. Starlink likely already had access to export to the Kingdom, so that's who was billable.

In regards to the supply aspect, the guarantee of military operation means only binned units are supplied. They likely didn't have additional binned units, so the UK provided their units.

>Why only these units went offline?

The war isn't in the news as frequently in the USA. So SpaceX wasn't receiving the advertising as desired. So they defaulted the account. There would be substantial negative press if they terminated service for civilians, so they didn't.