The bad English used in this mail is quite obvious, even to non-native English speakers like myself. You would expect that someone who is supposedly the head of some department in some serious company in London, would (be required to) speak English properly, even if he's not born in England. The body of the mail is on a "spam-bot literacy level".
Also the name
John Mark sounds weird. Something like
Jack Joe from USA
I know that the prospect of a new client sounds exciting, but sometimes it's better to be cautious. I also don't think this is a joke, but some sort of (automated bot?) phishing attempt.