Hi Steve,
well I can think of just one off the top of my head - ?D?Ye Ken John Peel?, written by local person, became popular, went into print as broadside and in chapbooks (Fordyce, Newcastle) and then published in local book 1866, Stokoe &Reay 1893, National Song Book 1905 etc etc. Later collected from oral sources by eg Williams.
There are other regional songs which circulated in a similar way, which presumably you wouldn't include in ?the national corpus?, a term with which I am unfamiliar in the folk song context. I?m sure you?re right that proportionally more, possibly many more, songs originated in the pleasure gardens and theatres (not all urban: there were plenty of small companies doing ?rural touring?, albeit often advertising the latest songs from London) and the songs composed by working class & artisan class (skilled workers) singers and musicians at Harmonic Societies and Glee Clubs.
I?m puzzled by ?the national corpus? you refer too though as I?m not sure there really is such a thing: there are too many variables - eg regional songs which become national as opposed to those which do not, Scottish (more usually ?Scotch? in eighteenth century)songs which are in fact English for example, while those published by Sharp et al represent a relatively limited number of singers in a few selected locattions, eg in my area, none of the collectors who came here ever went to a hunt meets so missed out on 30% possible Cumbrian songs. Can of worms warning!!