The key issue is proving the transfer of money was a loan and not a gift. If you can prove it was a loan then you will be able to enforce it against your father .
The difficulty with attempting to register a charge (ie. called a "restriction") against the property now is that as you would not have the consent of the propreitor you would have to show that you had a "sufficient interest". You would have to demostrate this to the Land Registry and they would require evidence (more than just evidence of the transfer of monies).
You may have difficulty obtaining judgement against him now because he has not yet breached you agreement with him, though he will do when he sells the property if he does not repay you.
Instead I think you are probably better off getting a local solicitor to write to him and asking for him to consent to a charge being registered and warning that you will sue him via www.moneyclaim.gov.uk if the property is sold and you remain unpaid.
At the same time you can ask the solicitor to register a unliateral notice against the title by lodging form UN1 and giving details of the agreement. This would give you notice upon the proprety being sold, at which point you would be able to issue and provided you can prove the agreement be confident of receving judgement. You could then enforce this judgement against him, possibly by applying for a charging order against another/new property of his.
It's pretty sharp practice from your father if you don't mind my saying.
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