Everything credible that I've read seems to be 100 per 100,000 for tier 2. Which does not place everywhere into that zone, and would at least give the majority of suburban areas a chance that they will dip in and out of harsher measures (though my area up from 30 to 80 this week, it has fluctuated quite a lot week by week). I have not seen anything indicating where tier 3 will kick in but perhaps will be > 200 per 200,000 as there are fairly few areas at that.
It needs to be proportionate, even Nottingham at 600 per 100,000 is only 0.6% of the population.
Case rate of 50 bringing in no household mixing would be far too extreme I think. That's 0.05% or 1 in 2,000 people.