I could elaborate, but it's not necessary. Scam.
You didn't talk to the owner. The person (who doesn't live in London) is a scammer.
I know the rent on the property seems too good to be true. Guess what? It is. That's the bait.
If you're going to use a Realtor (which is a good idea) for an apartment, then work through a Realtor, not directly with someone you only "know" via e-mail and wants money up front.
Hope that helps.
Hope that helps, let us know how you make out
Where did you find the info about that rental?
Check out my blog post on the subject: