What about when the rest of the country is that high?
afaik, The decisions depend not just on cases/100k but also #hospitalised, and whether elderly people are heavily counted among cases.
I dunno everywhere in country, but we are Tier 4 and ~190/100k cases. We hit Tier4 partly because this is the oldest (median human age) local authority in UK so we are high risk if things take off. We are nowhere near 800/100k and although local hospitals are busy they aren't in high peril (yet). If 800/100k is the simple threshold being used, then well done to decision-makers for using a consistent threshold.