I was born in Grimsby and learned 'Three Score and Ten' when I joined a folk band there in 1971. The leader of the band, Keith, used to sing it and assured us that it was about an actual event in the late 19th century ( he knew the exact date) when, as a result of severe gales along the East coast, many ships were lost, a large number from Grimsby. Keith had a family connection as one of his ancestors ( possibly his great grandfather) was lost in the storms.
However, he was also fairly certain that it was not traditional but was written as a music hall song, possibly in London.
Coincidentally, I used to visit the pub at Bradley Crossroads quite regularly. The "Songs Worth Singing" poster must have been put up after my time but I suspect that R G Brooker is Bob Brooker who used to be involved in Louth Folk Club at the time. Louth Folk Club was started in about 1970 by non other than Keith, the leader of our band. What a small world this is!