Seems to me they keep quoting benefits as "access to airspace". Well if they are hell bent on mandating transponders, why are they not considering the option of mandating mode C the minimum outside, class A (or D at worst), then its up to individuals whether they see the benefit of airspace access as worth the upgrade to mode S. TCAS on the airlines and fast jets (assuming the RAF put them on) will still prevent those collisions with Mode C.
Also they say:
"The introduction of SSR Mode S technology to replace existing SSR Mode 3A/C transponders would be an essential step in improving the efficiency of the SSR 1090 MHz ‘Reply’ frequency, and hence safety. Implementation of SSR Mode S would be the only way to permit the UK to increase the carriage of SSR transponders on aeroplanes and helicopters above current levels and meet the ICAO Annex 6 pressure-altitude reporting requirements. The withdrawal of SSR Mode 3A/C technology would also ensure that the full benefits of the recent investment made by operators in collision avoidance systems, SSR Mode S radars and the future ATC ground environment can be realised."
But don't back this up with any evidence. What is the evidence behind this, especially the bold bits?
I think they have built the case for this like a stack of cards, but with key cards missing.