The situation here is the same in Scotland as it is in England. Ultimately, you have no more right to look after your parents affairs as children than the next-door neighbour does. Hence, the lasting power of attorney. It is called a Lasting power because it can last even when your parents become mentally incapable.
If you were to leave it until they were mentally incapable (if ever that happens) would then have to apply for Deputy ship over their affairs which is much more long-winded and expensive and hence, it is better to apply for Lasting Power of Attorney now rather than apply to become a deputy at some stage in the future when they no longer have capacity.
Can I clarify anything for you? Please rate the service positive so that I get paid. We can still exchange emails. Best wishes. FES.