Regrettably, I think you’re onto a loser on this one. Unlike the United States Geological Survey (USGS), whose mapping data is open source due to the terms of their charter from the US Government, OSGB are a UK government organisation who agency status requires them to make a profit.
While technically, it’s our tax money that’s allowed them initially to generate the vast majority of the base mapping over the last bucket-load of decades, these days, they get a large percentage of their money from licencing the data commercially. Given the vast amounts of money this would cost the government if they made this data open-source, I can’t see it happening any time before there’s an exothermic reaction in hell 
