A couple of other points to mention re Scottie's excellent research.
6) In order to preserve market analysis all identifiable diversions are reallocated to the point of intended operation.
In fact, many perhaps most airports seem to claim diverted passengers as their own. For example, if a flight from A to B is diverted to C, C will invariably show the passengers as its own as will the intended arrival airport B because the passengers will eventually arrive there either by road or by the flight hopping over from C to B with the passengers later.
The other point concerns tables 12.2 Domestic Air Pax Traffic Route Analysis and 12.3 Domestic Air Pax Traffic Route Analysis by Each Reporting Airport
Table 12.2 only shows one set of figures for each domestic route whilst 12.3 shows them from both ends. They always take the London end figures for the London airport routes and for the remainder of the country (ie all non-London routes) they take the figures of the first airport alphabetically.
Why the CAA even bothers to show both 12.2 and 12.3 because the figures for each route should agree whichever 'end' shows them. But they don't - often.
At the moment because of low totals the discrepancies are much less for most of 2020 often by just one or two passengers, but a look at 2019's corresponding month - in the examples below November - shows wider variations with some examples taken at random and shown below.
On the route between BRS and BFS, the total for November 2020 is 3,429 under the BFS airport figures but 3,430 under the BRS figures. November 2019 figures are respectively 23,885 and 23,891
MAN-LHR from the MAN end is 9,569 and from the LHR end 9,711 with November 2019 respectively 50,604 and 50,547
GLA-LGW is 3,449 from the GLA end and 3,451 from the LGW end with November 2019 respectively 52,982 and 52,880.
I've noticed these discrepancies for a number of years and they are usually quite widespread each month.
Given some other obvious mistakes down the years that have never been corrected means that as far as I'm concerned the CAA stats, whilst broadly accurate, can never be taken as completely so.
As far as BRS is concerned for some reason it doesn't count certain types of passenger in its own figures and carries a note to this effect on the stats page of its website - under 2s are the only ones that the airport ever mentions but by implication there are other types as well. This means that, depending on season, the monthly BRS passenger figures can be anywhere between 5,000 and 12,000 fewer than the CAA's - 90,000-100,000 fewer annually.
This has been happening for many years but why it is done I have no idea. Most airports like to boast as many passengers as they can but BRS always quotes its own lower figures in press releases.