I've worked in a range of non-London offices across England. They were all exactly what you suggest. People start when they please from 0700 to about 0930. They go home approximately 8 1/2 hours after they started, sometimes a tad more sometimes a tad less, and over the course of a month it evens out.I wonder if there might be an element of London-based vs. not-London-based in all this, because of the typically longer commutes around London compared to if the office was in a Manchester suburb where most probably commute by car and typically with journeys of half an hour or less? Even the rail timetables around here are based around 9am starts - for instance the first semifast London service from Bletchley is the 0701 arriving at Euston at 0738, which would to be fair get me to my office by 8 (just, but certainly not ready for a meeting!) but if heading to the City proper would probably be pushing being ready for a meeting before 0830. There are stoppers from around 0300, but with those taking just over an hour you're talking needing to leave a lot earlier to get into the office just a bit earlier (though some may choose to do so now it means it's off peak under Oval contactless payment).
I've never worked with or known anyone who consistently worked short hours, and that includes the full range of people who started at 7 vs half 9. Meetings are tolerated by everyone at 9, as they're booked in at least a day ahead so people know what time to get in for. They're preferred at 0930 or afterwards, and could be arranged for before 9 if business critical. People wouldn't send meeting invites at 0800 without asking first, or at least giving rationale why that's the only time it can happen. If the rationale is there then people will accept it.