They happened in US, Japan, & some other parts of Europe.
They will NEVER happen in UK, I'm pretty sure of that. UK is horribly deprived when it comes to Pokémon
Unless in the next few years, Nintendo decide to do one in UK, but I'm doubting there will be one. Just so you know though, it is perfectly possible to have legit Mew & Celebi.
i got a mew from toys r us liverpool in the uk