I liked the idea that was being touted a few months back, for giving everyone a smart card to register their carbon usage for driving and flying. As a non-flying non-driver I would remain well in credit, and could then either flog my carbon to some greedy gas-guzzler or sit on it (probably the latter).

Haven't heard any more of the scheme, though. I suspect someone worked out that limiting the freedom to fly out of the UK would simply lead to a boom in trade at Orly.