Not decrying the effort put into that site - or the thinking behind buying British at all.
But we need to be clear that we're only talking about *manufacturing* in Britain. Quite a different thing from British *ownership* of those businesses, and quite another thing from whether the companies involved pay British corporation tax etc.
Not being an ar$e here. Yes, by all means support British manufacturing. But If the profits achieved from that manufacturing are abstracted abroad through overseas ownership (or tax haven voodoo), then it's still removing money from the UK economy ultimately.
Oh dear, that maked it much more difficult! By the way, does "British" include all the UK, and any overseas territories, or just England (or England, Scotland, Wales)
So.
British owned businesses only.
Businesses paying tax in the UK.
Factories in Britain (staffed by British people?).
Using British raw materials.
That wipes out many if not most possibilities (things using any imported material, for instance), and precludes many things which are actually British from being included (because, for instance, they're made or sold by foreign-owned concerns, even if they're paying UK tax and making or selling British stuff here).
I think we should in fact allow a rather less strict definition. For the above, one could substitute almost any country's name for "British", and it would be difficult to find things applying there, too!