Ah yes, I remember watching this on British television as a child - a definite classic :)
The movie was (loosely) based on The H.G. Wells story. Jules Verne wrote "From the Earth to the Moon" which was also made into a movie.
Between the two of them, they sent English fops to the moon, around the world, under the sea, back and forth in time, and to an island much like the one on "Lost".
There are several sequels, also very entertaining, and a prequel (Beware of the Mouse) which explains why the Duchy of Grand Fenwick eschews modern weapons (ie anything using gunpowder).
The movie of the original, while in itself not bad, follows the Hollywood tradition of taking enormous liberties.
Sigh. I think the cold I'm getting over did something to my brain.
The book that has Grand Fenwickians going to the moon is, logically enough, The Mouse on the Moon, the first of the aforementioned sequels to The Mouse that Roared.
Sorry for being pedantic...but if the Roman Empire hadn't fallen, then in fact you would have Roman Fops on the moon, there would be no English floppy-haired fops feasting on cucumbder sandwiches and drinking Pimms.
Instead you would have gladiatorial fights in zero gravity on the moon.