Better to say you can't take anything than to have to turn a/c away at the last minute.
I can see where we are misunderstanding each other here. I have no objection to an airport advising that it
can't accept diversions, eg. because it is already full, or due to a closed runway etc. In this situation, advance notice can only be a positive thing. The difference arises when the actual word we should be using is
won't. That is a different matter entirely. MAN's stand availability is not 100% bulked-out. Busy, but not full. And whilst some handling agents may indeed be working at capacity, I cite again the example of Jet2 who self-handle at MAN. If they want to divert their own aircraft in and feel they have the means to self-handle them, why is a faceless airport official pushing out a piece of paper two weeks beforehand removing their discretion to do that? It makes no sense.
A dynamic diversion policy offering discretion to operators is what is needed here, and it is the way things are done at almost every other major airport. A discretionary policy does not mean the airport becoming swamped, it means accepting what can be accommodated and refusing only those which really can't (as opposed to
everything, by default).
Most diversions arise at short notice anyway, so airports and agencies do not need to know the specifics days in advance, nor is that possible. A broad seasonal diversion plan should cover the essentials at this time of year.