My parents live in England. I live and file taxes in the US. I want to make them an interest free loan to buy a house. Payable at death / house sale

I considered purchasing it as joint tenants in common but that triggers stamp duty on the entire purchase as I’m a non UK resident. I thought they may be able to amend their will to represent the loan / share I have in the house upon their death. I don’t want for them to incur any taxes, I’m purely trying to enhance their living arrangements. I have one sister. I have as of now been unable to find a solicitor to help me think it through from a legal / tax / inheritance perspective. The amount I want to loan is in the 6 figures. Thank you for any thoughts you may have.