tawtee

Good morning all I wonder if someone can help. My son came to me today as he spotted a mistake stamp on his lunch money! We look at coins once in a while but no nothing really. It is a £2 brunell coin in nice condition ie hardly any marks. But the words two pounds are only half stamped. We looked around the Internet a bit and values are all over the place. So my question is.... is this worth anything? If so how much? And how does a school boy go about selling it? Thanks Tee