F3G
It is the tax on AVGAS that helps the government to balance the books and leave AVTUR tax free, that helps the airlines pay the nav charges that pay for the system that the government franchises.
So GA users do pay something for the system. Whether it is precisely hypothecated or not is irrelevant,in the same way that National Insurance is collected, but not hypothecated to a pension fund.
Sorry, but seeing as the CAA, NATS and all other sides of the aviation industry are self funded (no government assistance / subsidies) and haven't been since NATS was sold off your logic is flawed to put it mildly. My comprehension of your post was, therefore, correct.
If they have no input into government coffers then, by definition, they have no impact on balancing the books (AVTUR tax free is just that, it's not a subsidy, therefore it just means the government is looking elsewhere for financial input).
I'm not Gordon Brown, and I'm no economist. However, common sense dictates you can't include something you don't have in an equation. (Or maybe I'm getting it wrong and you can, hence the prevalence of the debt embracing society
)