According to the source, the event will be strictly to show off the Windows Phone Mango running HTC Eternity and Omega smartphones. According to the rumor, the Eternity will be a Mango smartphone with a 3.7-inch screen that will land at AT&T. The Omega will be a larger screen smartphone for T-Mobile USA.
That Omega is said to have a 4.5-inch screen. The thing that throws me a little here is why would devices destined for the US be shown off first in London. It would be a safe bet that the smartphones will hit UK carriers as well. Perhaps more details on launch date and pricing along with carrier info will land at the event.