The Strasser scheme is AIUI intended for weather diversions, but obviously would also cover a Mayday situation which is much more serious.
There have been suggestions that a proportion of pilots have been using this to avoid landing fees, which is perhaps why some airports took a long time to sign up. Big airports like London Gatwick were
AFAIK not asked to participate, presumably because they would be sure to refuse (with landing+handling fees around £500).
I am not aware of any deal or agreement or official policy when it comes to a Mayday to one of these big ones, and I would expect to be charged the full whack if I landed at say Gatwick. But one might be lucky...