Considering you did all this voluntarily I don't think there's anything you can sue your employer for. They set the terms of your employment, you agreed to them and have been carrying them out, and they pay you. As far as everyone is concerned everyone in the contract is fulfilling their end of the bargain. The only thing that's wrong is that you're not happy with what you're being asked to do and yet you're still doing it.
The short term solution here is to talk to your manager. Tell them you're unhappy and that you're not doing the job you signed up for. The longer term solution is to find a new job.
Also the jurisdiction of my contract is UK...
If there's a jurisdiction clause in the contract that you signed that states any dispute will be settled in a UK court then this is correct, but if there isn't then either party can nominate where disputes are settled (usually this happens when the contract is signed..) However if you try to sue they'll immediately terminate your contract/employment which doesn't sound like it'd put you in a good position.
Also... you're not being "treated like a contractor". If you are a contractor that doesn't mean you have to put up with this level of bullshit. No one, contractor or employee, should take crap like this in their job. Don't stand for it. Speak up.