As I know he NEVER signed in English. Why I think so? Because I never heard about it + it's very strange (I mean it's USSR, Gagarin signed only in Russian, because he knew only Russian). + I didn't found any information about it in internet (even sample). Yes, Gagarin visits many countries, that's true, but it doesn't mean anything. However, this handwriting in some places looks like on Gagarin's handwriting (especially second "a" in Gagarin), but it's very easily to fake (it's only few letters). I believe that it's a forgery (I'm almost sure). I'll be very surprised, if I'm wrong :)
Thank you for the reply, I obtained the signature from a person who would be the last person on Earth I would think traded in forgeries, I should have realised that he only spoke Russian, why would he sign in English.