He has cultivated his image to be a loveable buffon. I think that's a huge reason in him winning the Mayor of London race.
He's seen on TV as a bit of a posh, but dim-nice guy who is bumbling about on TV making gaffs and jokes. His hair just adds to that, it looks like it is fairly unkept and its just part of the image he presents to the media e.g ''Oh god! What's he going to do next?''.
I don't live in London so I don't know well/bad he's done as a Mayor but I'd guess that a good number of votes came from people who thought that he came across as a plum so lets vote him in for a laugh.
There's bound to be clips on youtube of his gaffs and interviews. Just search 'Boris Johnson'.
As for being a NW1 at 40-50. There are tons of folk who have strong hair. My neighbour is 78 and he has a slick-back NW1. I even joked with him that his brown chinos, cardigan and hairstyle is back in fashion and he was setting the trend.
The guys tht you mention have 'given up on life. I don't think they hae done that but they have probably accepted that their hair is not going to get any better. Many of them probably don't care what others think either, they'll have mortgages, wife, kids, job etc other worries more important than how Joe Public perceives them on the high street.
I'm 28 and I'm trying to maintain my hair and of course I have an issue with going bald after all I post on here, but even the last year or so. When I began balding, I was anxious and nervous about how people would view me with my hair but as I've gotten older, yes I still try to look reasonably good but I'm slowly beginning to care less about what others think and what/how I feel about myself.
I also started balding at 21-22 and out of my close friends, I felt it very much a s they all seemed to have thick NW1s ad were all in good shape but roll on 7 years, some of them have lost hair or it has thinned out, many are out of shape and in general are aging, maybe tha's my bitter side but when I seen myself balding before I seen it as un-fair and now I see others around my age having similar issues, it seems to have softened the blow, a bit.