OV
Unless the BBC has started rolling out HbbTV, the overlay probably is an MHEG prompt, just one that's be programmed to only appear on smart TV's.
MHEG doesn't actually have a way of showing prompts before you load a service (unlike Sky), so when you see a prompt it's actually an MHEG application that has loaded but is programmed to show only an overlay instead of a full service. As such they can programme it to use any button to trigger an action, green in this case, and any button to remove, usually green, or indeed none at all.
That's a 'smart' tv overlay. I know on mine you press exit to clear it. Green still removes MHEG prompts.
Unless the BBC has started rolling out HbbTV, the overlay probably is an MHEG prompt, just one that's be programmed to only appear on smart TV's.
MHEG doesn't actually have a way of showing prompts before you load a service (unlike Sky), so when you see a prompt it's actually an MHEG application that has loaded but is programmed to show only an overlay instead of a full service. As such they can programme it to use any button to trigger an action, green in this case, and any button to remove, usually green, or indeed none at all.