Being a lawyer.......ahem...
You can break, but you will be liable for the loss on the contract, that is the 3 months rent that is due. You could be sued for that, it might just come down to a question of whether it is worth the agent pursuing you, or whether they are willing to write off the loss.
Often, being honset and reasonable can help. Nobody ever does as they try and pull fast ones. I would be tempted to 'fess up that the rent and out goings are killing you so as to give the agent a chance of getting a new tenant. The flip side is that if you do a bunk you have already sown a seed in the agent's mind that you are not worth suing as you do not have the corn.
If you do get sued and lose you will get a county court judgment and have to pay it over time. It will stay on your credit record for three years.
These days it is a lot easier to sue for small amounts as the court have an online system.
If you do decide to risk being sued don't let a collection/bailiff company bully you too much.