Fuzzy,
The US has a long history of hypothecation in many areas. It would be very typical to impose a tax/fee to fund some activity and then ring fence the money to only that activity. The Americans have a view governments will spend their user fees/taxes on all sorts of other rubbish - unlike European governments
!
The UK government has a long history of flowing most income straight into the general fund. So a move to uniquely hypothecate Avgas tax is a very fundamental challenge to the Exchequer.
The number of instrument rated (not IMCr) pilots flying relatively low level is pretty limited (probably similar in scale to the number of moat owners) so the issue of providing low level airways is as C421 says pretty specialist.
In addition, I am not sure how many PPLs and IMCr holders are going to thank you for pulling CAS much lower thus eliminating their ability to fly at all for PPLs and without full control for IMCr holders in sub VFR (ie. with less than 1500m from cloud 1000ft above 500ft below and 5km vis when above 3000 ft) in all of this new class D or E.
Finally, to what the lower level airspace in the channel is used for, I don't know -- But the ATCOs do say it is busy. I have never seen any commercial traffic on any of the virtual radars at this level, nor have I seen it visiting Swanwick, and haven't seen anything on traffic systems while flying in the areas - but I only look at 0.1% or less of the total time it could be used.
To understand that the issue exists is worthwhile. For the non-flying ATCOs to understand the UK maybe ICAO compliant - but is different from what our near neighbours do - and therefore is more likely to be a 'surprise' for foreign pilots - is worthwhile.
However, in the list of priorities worth fighting for changing this part of the system doesn't even make the list to consider.