I know this is not much use to you, but I include anyway, if only to highlight just what a rip off London's airports are.
At Bangkok's new airport, the daily rate for short term parking, with no limit on the number of days, is Stg3.50, which gets you a space within 100m of the terminal. The long term rate is Stg2 per day (with a maximum of Stg8.60 per month), but that is 2km away.
I don't think it highlights anything. Remember that the average wage in Thailand is quite a bit lower than it is in London. It is nice as a Westerner to be able to go there and enjoy a cheap trip because of the exchange rate not tallying with this, but it is also very unfair to the people in that country.
I agree that airport parking is a rip-off, which is one reason why I mostly travel by public transport to/from airports whenever possible. However, I don't think you can judge it based on other countries unless those other countries are in Western Europe, the US or anywhere else I missed which is on a similar economic footing.