Quite VG. I believe the expected attendance at Twickenham (UK IC venue) is around 50,000. This in a stadium that holds 82,000. The catchment area is the western half of London and several counties, with a max driving distance of around 280 miles.
The population of the catchment area is conservatively 14,000,000, with perhaps 40-45,000 witnesses.
There will be several thousand international visitors. (I guess 2k but I don't know)
There are some good reasons why they will not fill the stadium, not least they tend not to use areas behind where the staging is but nevertheless it is significantly below capacity. The whole thing is being presented as a major, major logistical operation as if the area has never seen this many people in it's history. The fact that the stadium and the surrounding area has to cope with another 32,000 people for major rugby events is ignored. True, this is for 3 days and not one afternoon but move a few miles up the road and you have Wembley, capacity 90,000, that copes with major sporting and music events regularly.
I am not suggesting there will not be disruption to traffic, nuisence for the local residents etc. but the local area is used to these things and on a bigger scale.