How much does it cost to stop a mobile contract?

I have my blackberry on t-mobile contract for £25.00 a month. The contract was for two years and is ending this May, I want to stop it earlier though. Will I get charged the rest of the payments altogether or will there be a small charge because there isn't much time left.

I've had enough of blackberry's and I want to get the iPhone 5 now.

There's an Early Termination Fee (ETF)

Depending on your contract clause, if its a flat fee or subject to time remaining left on contract… ETF can range from $60-$350… Plus prorated final bill charges… So the final bill will be (for you since you have a $25 plan) anywheres from $70-$370

Walk into whichever shop (tmobile, O2, vodfone) sold you the original contract and ASK.
If you are staying with the same provider, they may waive or discount the early upgrade charges.

Usually, it's the full cost of months remaining.
If you hate the Blackberry Handset, sell it to pay off the contract (Cex gives fast cash-in-hand)