A parent giving a loan to a child to buy a house is not unheard of. What is unusual is that the contract says you cannot sell the house nor repay the loan early. Usually a contract would detail how repayments are to be made, interest (if any) and any early repayment penalties.
what you have is a loan from your father to buy your house. You also have a mortgage, which is also a loan to buy your home but with the home itself effectively as collateral. The mortgage will be recorded at land registry for the bank to protect it’s interest in your home.
presumably, this is not the case with the loan from your father? Is there a charge registered against the house from him?
importantly, when buying the house, you will have needed to illustrate to your solicitor where you got the £200,000. did your father sign a document that this was a gift? How did you show where these funds came from if not declaring as a gift?
if you did not declare this as a gift, you may have committed mortgage fraud.
Going back to the loan from your father, the terms are classed as unfair contract terms and would not stand up in a court of law - which would be the only way for your father could try to enforce the contract you have with him.
if there is no charge from your father registered in your house, you can sell without any input or even giving notice to your father. If there is a charge registered, you need to pay him back to remove the charge.
You could simply repay loan and show proof of repayment to have charge removed. You have his bank details, just make the deposit noting “full and final payment of house loan” (include charge number too).
if there is no charge registered. You could also remortgage your house with the bank to include what you owe your dad and just deposit into his bank account also noting as “repayment of house loan”. He is not going to try to sue you in court for repaying an unfair contract.
however, I suspect you have borrowed more than the bank would permit (your existing mortgage plus £200,000 loan from your father). If so, then you’re complicit in mortgage fraud. You need to think carefully what you want to do about this. Perhaps repay your mortgage faster to reduce overall debt (debt if your existing mortgage plus your loan with father) so that you can qualify for just one mortgage (one loan) for your house purchase, sell house to achieve the money to repay your dad and get another mortgage for entirety of your onward purchase. Just cut your father out of your finances.
your father is unlikely to do anything about any step you choose because he is being dodgey tax wise and it would cost him a lot in legal fees. Thus is not a county court action due to high value plus no one is going to represent him with such an invalid unsecured loan document that was repaid.