Normally messages from the British Embassy Visa section worded in that way mean just what they say. The visa is being despatched.
These are automated messages, and of course may have been sent by mistake. So the only way to be 100% certain is to hold the passport and to see the visa with your own eyes.
I've not previously known any such errors though, so I would say congratulations are due.
What visa did you apply for?
