The problem with the CAA's approach is that is essentially reactive, rather than proactive. By waiting until HAL has put forward its (expensive) scheme it makes it very difficult to achieve any meaningful savings. What the CAA really needs to do is to say to HAL that its maximum charges can be no more than £x, based on realistic benchmarks, and then challenge HAL to come up with a scheme that is consistent with that price.
Otherwise it's just a cost-plus approach, which inevitably leads to gold plating.