What noises do you make to say "no"? In your opinion, does this vary from one part of the UK to another? It seemed to me, when we traveled westward from London, "no" got longer and longer, until people seemed to be saying "no-ah".
For a US example of what I mean, in Appalachia and the Ozarks, folks might say "nope". Yup, just like in the cowboy films. Of course they say "no" in the ordinary manner as well.
If Bodie and Doyle are doing a bit of Scouse or Brummie, how might they casually say "no"?
It varies extensively from location to location.
In Bristol or Somerset it might be as you say, an elongated "Noh" but we've also picked up the London "Nah" or the US "Nope".
In Lancashire it'll be more like "Naw" and Newcastle, "Noo".
Scouse is difficult to put into phonetics. The best I can do is "Nehw" but it even varies from district to district.
Brummie would be "Now".
Many UK regional accents can be all but incomprehensible to other regions of the UK, so to an unfortunate world traveller they may as well be another language.
Some phrases are uniquely Bristolian. Only here can you buy a teeshirt with the phrases "Alright my luvver" or "Gert lush". 
Great post from Eccs!
Don't know if this is any help - Scouse dictionary
Or Talk Like a Brummie including some audio clips
I have the London "Nah" that Eccles refers to though I can drop it for more formal conversations.
It just struck me that a lot of the time I say "not really" instead.
"You hungry?" - "Not really"
"Is it cold out" - "Not really"
You'd think that 'no' would be quicker.
Fun answers, all. Thanks so much for replying.
I do wonder how the hecque "nope" evolved. Must explore that a bit.
Of course, if you're David Walliams then your answer would be a banal "The computer says 'no'"