would be great to at least know the reasoning behind Hughes comments, but think it's a bit late now to ask him

My opinion is that yours is too small to be a serving rummer, or serving punch glass............ punch glasses, or at least those I've owned, have always been a lot smaller than this one ........... the ladle image is intriguing - perhaps they 'ladled' syllabub into this one.
Knowing as we do the art of producing glasses c. 1750 - 1830 ish, they gave a lot more thought to fine glass ware with thinner structure than the crowd that came after William IV, and the Georgians had tankards with more finesse than the Victorian heavyweights, plus many more designs were 'blown' since the advent of mass produced mould made items was a little way off.
I think early tankards were made both with and without handles - straps and round section. As it's cracked I'd chuck it and move on to the next gem