The Rapha Festive 500

So if you’re in London, that covers the London Boroughs but you can’t go elsewhere. If you’re in Kent you have to stay in Kent and can’t go across to a different tier 4 area and so on

Is this written down anywhere official?

(Also, the rules continue to make a concrete distinction between travelling to somewhere and passing through it during exercise, so arguably riding outside Tier 4 is allowed as long as you don't stop)