I live in a tier 2 county, but only about a mile and a half from the border with a tier 3 county. I go into the tier 3 county once a week anyway for work.
According to the government tier guidelines, you may go into a tier 3 area 'when necessary, such as for work or education'.
The branch of Aldi I go to weekly (and went to throughout the first lockdown) is just into the tier 3 area. Would I definitely be breaking the rules by continuing to shop there? Food shopping is, after all, necessary. But I could go to a further away supermarket in the other direction if I had to.