If there are specific things that the landlord is in breach of you may have a case for terminating the contract- but you should be reasonable and give the landlord the opportunity to remedy any fault. It sounds like the honest  reality is that you want out for your own circumstances not really for anything the landlord has done

So you are probably tied in to the contract to be honest-

my advice is:  negotiatiate with the agent/landlord

help: i.e if you can find suitable replacement tenants so the agent/landlord is not going to lose out on rental they are likely to be more amenable to agreeing to terminate your lease

Alt you may do a deal with them such as you pay a couple of months rent to cover the landlord for potential void period while they remarket in return for them agreeing to terminate your contract.

try and come up with win/win solutions to the problem - most people are reasonable