In fairness, attractions can only do so much to enforce the guidance given from the government. Realistically, no attraction is going to check the address of every person, and then check what tier that address is in. It's the same for restaurants: no restaurant is going to be actively checking that every person in a group is from the same household.
This conversation is presumably coming from Theme Park WorldWide's visit to the park, despite the fact Shawn lives in a Tier 3 area and the park is in Tier 2, with his justification being 'TPWW forms his sole income, so he's travelled for work'.
If you live in a Tier 3 area, you are told to avoid travelling, but you are not banned from it. So Shawn/TPWW aren't breaking any rules. Attractions and venues can, in theory, stop them from entering under their own T&Cs, but almost definitely wouldn't unless people were causing issues.
Ultimately, it's a bending of the rules, not a breaking of them, and you could argue it's morally a grey area. And it shows how easy it is to exploit the government's recommendations, and how important it is for every single person to play their part, and use their own judgement and common sense.