Everyone else is to blame.
Shopkeepers used to sweep their own bit of pavement every morning, and householders the same.
In the meantime, volunteers are going to the beaches to help clear up rubbish.
If you aren't part of the solution, you are part of the problem. When you walk past litter, do...
In which case I dispute your description, unless, as I say between 2pm yesterday and 1pm today, people went out with their dogs and litter to make sure 'it was very heavily laden'.
I think you could be exaggerating.
Was that today? I walked along half of that route yesterday, and it wasn't as bad you describe. Maybe the litter louts and dog walkers all went out overnight.
Incidentally, there are street cleaners in Brighton, because I've seen them.