Is it a crime to 'forge' someones signature on a document when acting wholly in their best interests?

My father has lost mental capacity. At present we do not wish to apply for power of attorney. He has savings in an account and is a non taxpayer and is not getting interest gross. THe bank needs an R85 form from him to ensure the interest is paid gross. I cannot instruct the bank myself with out power of attorney. He cannot sign the document or give consent. If I forged his signature on the fomr and sent it in it may result in the interest being paid gross. If this came to light what crime would i have committed if any? I am thinking it is false representation but i am also thinking there also need to be fraud and so a gain to me in connection with this for it to be an offence? I also thinking that such a case would not be brought to court anyway? Obvioulsy we are making all sort of decisons for him every day on his behalf as he cannot give consent - such as choosing/buying clothes and food. Choosing where we take him - all in his best interests. The only diffeence with this action is we need his signature to make this action happen and the banks offer no alternatives except via power of attorney.