In the classic Monty Python sketch, The Cheese Shop (video here), what bizarro accent is John Cleese channeling at 0:50 and 1:22? The point seems to be that he’s temporarily left behind his erudite, snooty talk, and is talking down to the shop keeper’s level in some way. Is this some UK version of a “hick” accent? Something else? I’d guess the quick cut to some yak herder (or what ever that’s supposed to be) at 1:23 is a clue, but I have no idea what that’s about (or its completely unrelated, MP is just like that sometime).
The script has it listed as “broad Yorkshire” and “Broad Northern” respectively.
Any idea what that means? Or why its funny, for that matter? 
You have already answered why it’s funny:
Apart from that, the accent in itself isn’t particularly funny, except in its very opposite sound to the rest of how he’s talking.
The Yorkshire accent is a stereotypical rural accent. It’s kind of like an American speaking in a broad southern drawl, or a Canadian talking in a Newfie accent. It’s supposed to evoke a stereotypical “unsophisticated yokel” kind of effect.
Presumably, the joke is as has been stated: he’s making a somewhat condescending attempt to talk down to someone else’s level.
thwartme
Not really specifically rural, or “yokel” - that particular stereotyped accent evokes in me, a southern Englishman, someone who dwells in an industrial town or city.
Yeah, the joke is that the shopkeeper doesn’t understand ‘posh’ but does understand ‘common’.
It’s a bit less so now, but back when MP was made a person with a regional accent could be seen as being working class, uneducated and lacking in sophistication (in other words, common). That goes double if it’s a Northern accent.
Not exactly hick, but the Southern drawl comparison would be a good one.