I've had a few 60 day visa's before, but the London embassy have never asked to see evidence of a return flight because that's not required. The airline may ask for evidence before they let you fly (although you don't need one) but that's a different matter.
I had that. It was very difficult and expensive to extend a policy when already abroad. You just have to bite the bullet. No good lying online as to where you are, as you won't be covered...of course. Not an answer for your query but when I entered on a 60 day visa last time, I bought a 90 day policy as the extra cost was minimal in case I stayed longer.