It does seem strange, I was convinced I would find it in one of my books. Searched the GMB but if there is one I don't think it was described as a tankard. It seems nicely made, with a fire polished rim though there are a few bits of ash within the glass.
Perhaps made for Ikea or another store or pub chain or ...?
Tried searching under the usual suspects, Whitefriars, Wedgwood and Dartington.
John