It could be that there's a doppelganger situation. The character in London has a look - alike in 90210, and the events that made people think it was impossible are actually related to this look - alike. Or, there could be some sort of virtual reality or dream sequence where the perception of the character's location is all messed up.
Well, perhaps there's some sort of time - space anomaly. Maybe a magical portal opened up that allows the character who is supposed to be in London to suddenly appear in 90210. Another possibility could be that it was all a big misunderstanding and the person in London was misidentified, and the real character is actually in 90210 all along.
The plot twist might involve a secret organization with the power to teleport people. They accidentally sent the character who should be in London to 90210. Or, it could be that the character has a hidden power or ability that they themselves are not aware of, like the power to be in two places at once, which is why it seems impossible that they are in 90210 when they were thought to be in London.