I'm sorry to say it really sounds like your have plagiarised my university had zero tolerance on both plagiarism and collusion any occurrence of either of these (if proved guilty) would see you kicked off the course if found guilty after graduating you would be stripped of your award best case is you may just get zero for the unit if you have enough points in other units you might still graduate though but most likely your screwed and won't get your degree , sorry but it's your own fault if you had referenced properly you wouldn't have this problem.
Just going to have to deal with the consequences, OP. I'm really sorry. Just own up to it and wish for the best.
You can't deny it, they have proof.
Face the consequences of your mistake, trying to argue a point you can't backup would be silly.
Do not admit to anything. Play dumb & don't say it was intentional whatever you do otherwise you can definitely kiss your degree goodbye. This site has some very useful info on what to do in this situation. plagiarism-defence.tripod.com
First thing to realise is that what you have is a criminal offence in the UK but the law is relaxed around students meaning you won't be arrested but you could have your degree either stripped or downgraded (if you are lucky).
This is different for international students though as once you are out the country then you are no longer under the control of UK law, but, this isn't a way out anymore as it is part of the agreement you signed when you started uni.
Best steps for this is to contact your professor and state that you Deny all charges against you and also say that you are not in any position to travel back to the UK (think of a good reason like money issues ect). Then, if he comes back saying they can 100% prove it you will have to claim ignorance, you tactic next depends on how your essay was laid out as you could say something as simple as "I forgot to put quote marks in" if it fits the situation. The UK government will not ask to have you sent back over as it is not a massive crime so you could play it safe and simply not return to the UK.
Just remember once you start denying it you can't go back on your word because things will get a whole lot worse for you.
You should find a suitable, remorseful response made by a famous person, then copy and paste that to your tutor.
When you submitted it via TurnItIn it gives you a percentage amount telling you how much is matched. How much did it say was matched? I've seen people with up to 30% matched but with decent citations throughout and they've been fine. For you to get called up on this I can only assume you've plagiarised a significant amount of you've been sloppy with citations. If you had just been sloppy with citations the likelihood of it being brought up as plagiarism is lower and a departmental meeting would be more likely.