Do the financial rights of creditors come before the sustainability vultures' rights of Mother Earth?

Sir, Tim Smedley writes “The UK has a legal obligation to meet 15 percent of its energy demand from renewable resources by 2020” “Sustainable jobs surge fades as wind farms face”, July 17.

What does that mean? Can you bank on it? If duly servicing UK public debt requires subsidies to wind farms to be cut and as a consequence you will not be able to meet targets… could there be a renegotiation? Could sustainable energy vultures hold out… and force a cut in all other energy supplies?

It would be really interesting if FT explains how this kind of legal obligation would rank with respect to other legal obligations.