In view of size, I'd have suggested a handled mug rather than a tankard, handled cup, or can - however, I doubt very much that you'll find a maker unfortunately. It's a good piece, and the ring of which you speak suggest a good quality lead glass, which combined with the shape of the handle, and the wear you mention, suggests manufacture perhaps somewhere around the 1820 - 50 period. Not necessarily British, only possibly - the diamonds might be chequered diamonds, but not certain.
Other points to look for re period might be small specks of black or white dirt/stones in the glass, and the colour of the glass should be a slightly grey colour compared to modern cut glass. Sorry not really of much help - let's hope someone else can add more useful information.