The Natural Philosopher expressed precisely :
I just got one
i wonder how they got my number

I suppose the service providers just provide a list of their
subscribers, or forward theses texts to all their subscribers.

Mine was an O2 number, now paying Plusnet which I think EE provides
their service.

I'm on meds for high BP, supposed to also be slightly T2, plus kidney
with a couple of appointments pending, but still no message from .gov,
NHS or my local surgery. The latter two definately have my mobile
number.