Go and get some legal advice on what you may be able to do to get out of your current situation. Make sure you see someone who is an expert in Employment Law and make sure you have a copy of the contract you are being held to.
you're an employee. You have a contract of employment. In this contract of employment, in exchange for their paying you, you have to give them notice of your intention to quit. Which you are now trying to renege on? Either resign, and see how long they keep you for, and honour your contract. As they used to say on Stock Exchange " Dictum, mayum pactum" Brutal, but then so is the real world....
I am more than willing to give reasonable notice - no intention of just running, but according to contract I can't leave until end of contract!
The contract is highly unfair and relevant compensation is well below market average. If I do leave any earlier, I am liable to substantial and unaffordable penalities based on my profitability. I'm just stuck with no realistic options.