It is Miranda Richardson herself, in an interview about this film, who postulated that it was a cross between Mel Brooks and an Ealing comedy. And to some extent she is right. It's not as bad as some of the critics have described, but you must remember also that, as Ben Walters wrote in the 'Sight & Sound' magazine review, this is meant to be a bad film.
It does have a number of funny moments and characters - Miranda herself as a bored Eva Braun, Anthony Sher as a socially-uncomfortable Hitler, Vic Reeves and Bob Mortimer as camp royal servants, and Harry Enfield as a parsimonious and small-minded George VI. And if you buy it cheap, you cannot complain about wasting money.
The extras are quite worthless, though, even the commentary by Christian Slater and Peter Richardson. But at least you learn therein that the movie was not filmed mainly in the Isle of Man as the credits state (something to do with funding by the Isle's film company perchance?), but was actually shot mostly in Devon - Brixham, Buckfastleigh (Dart Valley Railway), Dartmouth, Paignton, Plymouth & Powderham.
So long as you watch this film in the spirit in which it was made, you won't be disappointed. It kept us entertained for an hour and a half, and will do so again.