Please report dangerous drivers to the police - Roadsafe. Report to plod, not just whinge here.

This could be the new Pokémon Go.

Anybody know if it's worth handing images of people using phones whilst driving to the police? Can they prosecute or is it only if they are seen by the police themselves?

I doubt they can prosecute based on a single photo. It doesn't conclusively demonstrate use so could be easily challenged.

The Police can issue a fine on the spot because they get to be an expert in deciding whether what the driver is doing is distracting or not.

"
You can use hands-free phones, sat navs and 2-way radios when you’re driving or riding. But if the police think you’re distracted and not in control of your vehicle you could still get stopped and penalised.
"
Source: https://www.gov.uk/using-mobile-phones-when-driving-the-law

As for the first question, it might be a good idea in the light of the recent news articles and pushes to clamp down on it more. If the Police get more and more reports of people doing it they may decide that there's some merit in actually doing some enforcement.