I think you are referring to an enduring power of attorney when you mention completing a form in your original post. From my understanding this needs to be completed while your parents have legal capacity and their doctor is required to make a declaration to that effect. GPs, I understand, are generally familiar with the process. My understanding is that it is necessary for a solicitor to be involved in the execution of a power of attorney.
I understand that you may not be able to afford a solicitor but it may be money very wells spent. You can shop around for the price of executing a EPOA and it should not be that expensive as the legislation provides for a precise form of wording that can be used. There is also a very high cost of not having an EPOA in place in that any decisions in relation to your parents may be very difficult, if not, impossible to make. I am aware of one circumstance in which the gentlemen has dementia, no longer lives in his house and it not possible to rent it out (or do anything else with it) as no-one can make a decision for him.
I am not a solicitor so I'm more than happy to be corrected if any of the above is incorrect.