I added EG so that all UK airports had the Swissport handling agents. However, at Gatwick, I am only getting British Airways, easyJet and Virgin Atlantic and I'm guessing that every other UK airport apart from EGGB and EGGC won't have a Swissport option.
This indicates you haven't understood the scoring system. The PaintKit documentation, Page 7, says:
The Number of character of each code will define the score of that code in case of a match, and will be evaluated against all the codes of the other suitable vehicles. The vehicle with the highest score will "win" and will be selected for service. If two or more vehicles have an identical score, the winner will be determined randomly
So, what you did, was to assign a score of 2 to Swissport in all UK airports. Which means, as explained above, that Swissport will appear ONLY at airports where NO other operators have a score higher than 2.
So, in order to be sure you get an operator on any given airport, you must do one of the following:
1) If you have assigned a score lower than 4 (by using less than 4 characters for the code), check that no OTHER operators have an higher score
OR
2) Assign it a score of 4 on that airport. This will ensure the operator will appear on that airport.
Depending on the airport and the number of competing operators, #1 might be easier than #2, or vice-versa.
To make sure, I also reviewed the FlightSim entry for the aircraft I was using:-
'atc_parking_codes = _SWP'
Quoting from the PaintKit documentation, Page 7:
NOTE: only the Parking code matters, the airline code of the airplane you are flying doesn't affect the choice of the operator
It is very difficult to understand a manual which was written for a completely different version and of which refers to things which are not anymore applicable.
There isn't a single line in the PaintKit documentation that is not applicable to the current GSX version. The ONLY difference would be this sentence:
If two or more vehicles have an identical score, the winner will be determined randomly.
That might be "updated" as such:
If two or more vehicles have an identical score, they will be all listed in the operator selection menu, with a randomly selected default entry.
But this won't obviously affect any of what you are supposed to do to change the files, just make it easier to get the operator you want.