It's true that the RAF does not owe him a farewell party. I don't think anyone could produce an article from the Air Force Manual of Law or whatever that states he is to receive one.
However, what about the command at the unit or base? Legally, no - but are those people acting in the best interests of recruitment & retention? Wouldn't it send a good message if even a small soirée was held, thanking him for the service and wishing him the best in future endeavours?
How do you think this one small incident influences potential recruits for the services? If asked about joining the RAF, will Gaspode give positive encouragement?
Even if we take an extreme view and suppose that Gaspode was unpopular and that they were glad to be rid of him, a small farewell should have been organised by the unit - because it is the right thing to do.