Neither of them need to have great grammar but clear diction IS required. Winstone reads other people's words, he's not a writer. Same as Cheryl doing an advert - but the fact that one of her lines in her current advert is, IMHO, open to an unfortunate but hilarious misinterpretation purely because of her pronunciation proves she should polish her skills.
Nothing to do with being "posh", just being understood.
I'd hardly say Winstone's diction is clear!
Great diction went out with the likes of Alec Guinness!
I think Cheryl Cole has polished her speech quite a bit in recent years. Not that I follow her career closely.
One does not have to be "posh" to attempt to speak English correctly. Accents are fine as far as I'm concerned, it's when people say things that are grammatically incorrect that annoy me (for instance "I didn't do nothing"). It sounds like they never went to school. But people accept it from someone like Winstone because it makes him sound "hard" and "from the street". I'm sure if he started speaking proper English he'd be accused of selling out and his brand would be damaged.