If the tax came in it would remove the advantage in price and interest in JetA1 powered light aircraft will drop significantly. In the US there is very little interest as the price difference is not large.
AvGas at Cambridge is (was late last year) £1.04/litre, of which £0.28 is duty. That leaves about £0.76 as the base cost of AvGas. That compares with £0.49 for Jet-A1. At larger airports, I imagine the difference would be much greater.
The European market for AvGas is so much smaller than for Jet-A1 that the price can be hiked mercilessly, with little effect on sales volume. (Indeed a few years ago, before the suppliers apparently cottoned on to this, AvGas was cheaper than MoGas with duty included -- just a little more as a base cost.)
Even if a tax on Jet-A1 were introduced, it seems unlikely that it would be anything like the 28p/litre on AvGas.