Can I contact the the police and will they

Basically last week I bought an iPhone 7 Plus from the Facebook marketplace from a guy that dosent live to far from my town he is a common person and we do have mutual friend he sold me the phone for £310 and we met up in mc Donald's where I handed him the cash and he the phone but the phone is locked with his girlfriends iCloud and it says it has no service too which means I can't log in I've been trying to contact him and he keeps giving me excuses but says he will sort it out I told him I wanted to return him but he says no and I think they might have reported the phone stolen do you think by going to the police they can help me I'm so nervous and don't know what to please help

Document everything including the FB post and the messages you've had with him and go to the police.

Can I contact the the police? YES you can.

Will they help me? Probably not.
What do you expect?
Of course you documented everything and have a warranty and guarantee.

The term in law for you is caveat emptor
the principle that the buyer alone is responsible for checking the quality and suitability of goods before a purchase is made.

You purchased AS is WHERE is.
That it does not work as you think it should is your problem.
That you did not test it is your problem.

NO ONE must give you a refund.

Facebook marketplace you mean the pretend place with no address.

There's a reason that brick and mortar places still exist.

You could facebooklet your complaint to the facebookie place.
You could twitter away your concerns,
Others might repost and soon the seller has no future customers.

Did you expect the police to send off an email or what?

File a police report of theft. Odds are, he stole the phone from her, which is why it is locked. Care to bet that they've broken up?