Rationing is a completely irrational response to a shortage. Rationing is predicated on the assumption that it's fair for everyone to have the same allocation of a resource. Nations typically behave towards each other like toddlers. Within a nation, the politicians seem to treat citizens like children, where every child gets an equal sized piece of cake.
I'm not a child. Neither are you. None of us are (except our children, of course). Giving each of us identicaly sized pieces of cake ignores all sorts of issues. For example, we are carbon-based lifeforms. Some of us are bigger than others, are comprised of more carbon, and need more carbon to live. Or for example, some of us use our carbon to reduce our carbon emissions. I own 225 acres of land. It has a growing forest which is pulling carbon out of the air. I'm a hero -- shouldn't I get a higher carbon allowance? What if I can consume (emit) carbon dioxide (in the form of fossil fuels) to optimize my carbon consumption (in the form of woody growth)? How does the carbon ration account for this? Particularly in the UK where land ownership is far more class-based than in the US.
An equal share is not obviously fair; not at all.