uk – If I land in London the night of September 9th, will I be required to self isolate until September 22nd or September 23rd?

A strict reading of the rules simply says 14 days, and as such the sensible thing to do would be to consider that 14 full 24 hour periods, so if you entered the country at 8pm (for example) on the 9th of September, then your isolation period would end at 8pm on the 23rd.

This isn’t the sort if thing to skimp on, keep yourself and others safe.