If you head butted me and 'bent' my teeth back, yes, I would consider it GBH. I would think the law would as well but I'm sure someone with more knowledge of GBH will be along soon to answer your question. What do you think might be the charge if not GBH?
Headbutting someone so hard that their teeth are damaged could easily be GBH.
If you have done this to someone expect to be charged with GBH section 20.
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