We've moved on a little from this:
From: Mrrzy
Date: 21 Oct 20
Wherever you go, be bound by the rules of where you are. Seems obvious.

But it's not quite that simple. Wales is in lockdown (officially "Fire-break") but non-essential travel is not permitted. So I can't go to England to be covered by rules existing in a lower tier. (Unless it's for work, or essential health reasons)

It would seem to be "You are covered by both the rules of where you are from, and where you're going, so work on it being the more restrictive of the two". But even this isn't necessarily accurate.

The governments (plural) and local authorities do not make it sufficiently clear to be able to identify what the rules are, and whether people are breaking them.