As to Rik Mayall, he was also born in Essex but moved to the West Midlands when he was three. His character Kevin Turvey had a pronounced accent and I've always felt he retained a little of it whenever he spoke. Of course when he went to university in Manchester he met Ade Edmonson and Ben Elton. From such decisions whole careers blossom!
It reminds me an awful lot of the accent Joyce Grenfell did for her Eng Lit character: the wife of the Vice Chancellor at one of the Oxbridge universities. It might be a combination of accents. A bit of East Ham where he grew up and a bit of Cambridge from when he was studying English.
"Courage isn't a matter of not being frightened you know; It's being afraid and doing what you have to do anyway."