What happens if I break the contract?

I'm looking at iphone deals with pay monthly tariffs (in the UK).

Just wondering if I break the contract what is the fee and is it worth doing to get a decent handset?

It depends how long you've got left on your current contract, if its only a short period then it might be worth doing. But if its 6mths + then it will cost you dearly. If you want to stay with your existing operator they may let you upgrade earlier, and tag on those extra months at the end of your contract.

Lets say your paying £25 a month on your contract and you have 12 months remaining, if you cancel the contract you will have to pay for the 12 months remaining = £300

The fee could possibly be you have to pay the contract in full, it could also be you have to pay the full price for the handset. Also if you do break the contract say goodbye to your credit rating as it will plummet an you will NOT be able to get a contract for a long time or a loan, or a mortgage it is really not worth it.
Plus I'm guessing that you do not have enough money to pay for the contract so they will not offer you one, or will mean you'll have to pay the handset price or a portion of it.
Also why would you want an iPhone if you aren't going to pay for data, calls, and messages in that sense you'll have a very expensive MP3 player that becomes an iPhone that can only make iMessage or FaceTime calls when connected to Wi-Fi.

You could try this though if you are eligible for a contract you could get an brand new iPhone 5s with it as soon as you receive the phone sell it on eBay for £500 and buy a used iPhone 4S £150 or iPhone 5 250 and then you keep the contract and have money to pay pay for it as well as having a good phone. That is what I did when my contract was due for renewal I already had the iPhone 4 on perfect condition so when I got my new contract I got antihero iPhone 4 sold it and used the money from selling to pay for my contract for a few months.