I prefer maths. We can use it here, because the march crossed Westminster Bridge which is 250 m long and 25 m wide.
Soldiers in formation march with spacing of about 1m in each direction. A number of these marchers had placards, etc, requiring mire space. Some may have been packed in tight. We can use the soldiers’ spacing as a reasonable assumption.
No one walks at the curb. Leaving half a metre on each side, assume 24 marchers in a row. If there are even 1,000,000 marchers, that makes 41,667rows. With a metre between each row, that is a line 41,667 metres or 25.89 miles long.
Soldiers march at a rate of about 2.5 mph. It would have taken a line of marchers over 25 miles long over 10 hours just to cross Westminster Bridge.
Who amongst the million plus believers thinks that it took the marchers over 10 hours to cross the Bridge?
Credit to the PP who said that this analysis was done somewhere on social media. I wish I knew who had the idea first, so I could credit them.
Note: as has been said here, it very easy to blur the different amounts of space people take up sitting, standing and moving. Hence the counter-intuitive Wembley phenomenon being discussed.