That's an excellent point robin. The CAA is also supposed to adhere to the 'user pays principle'. By their own admission, it's only the airlines who will be using the "new efficient" ATS who benefit, so the airline should pay. If the cost to equip the GA fleet is as little as the CAA claim, it shouldn't be too much of a problem for the airlines to pay for it. And they can look at it as a deductible investment towards future money saving efficiencies.
Of course why mention ATS efficiencies when the stated objectives are only safety based? It's a BS way to sneak through a financial burden on GA using a BS safety argument, the financial benefit of which accrues to the airlines. We can see who regulates the CAA.