You live in the UK which is lucky. We have 4 ISPs here in Australia and every single one of them blocks port 80.
Very similar thing in the US - most ISPs in the US block port 80.
I imagine the majority of ISPs block port 80, but of course there are exceptions like few UK ISPs doing so.
It may not be a 'universal problem', but it is definitely a problem that affects a very significant amount of users.