Both working class. Both Irish. Both the most interesting, and vital, person in music of their respective decade.
You are definitely onto something with your point about maturity. I think Morrissey turned 60 this year, Lydon must be older still and yet their career trajectories and the industry they inhabit mean they have to continue to act 'rebellious'. It's quite disturbing to see late middle aged men forced/feel required to act out in this manner and it demeans their talent and history.
I would point out that in respect of Brexit the position of both is representative of the position of most of their class and generation. Similarly, Trump speaks to a similar age and class in deindustrialised America where work, class and community certainties have been washed away leaving communities clinging to nebulous national identities.
There is however no rationale or reason I can advance for the Britain First stuff. I was gutted to read about it. I've loved and defended Morrissey for years but you can only conclude that the the misanthropy has finally gone far too far and turned him into a bitter prick.