I repeat, all the cap does is allow NATS to make assumptions. assumptions based on conjecture - unless they (the actuaries) have already decided that the RPI we use will be 1% or something.
Sorry to labour the point but I really don't think you understand what the cap does. The actuaries use an assumption on future pay rises, they have to do that because the estimation of future liability depends upon your final pensionable pay. That assumption has to be pessimistic so they use RPI + 1.5%. They also make an assumption on long term RPI which is based on Government figures for previous periods, again it is pessimistic (the figure was given at our meeting and is on the slide shown at the briefings but I didn't write it down so I won't quote it here). If NATS can guarantee that pensionable pay will rise by only RPI + 0.5 % the actuary can use that figure instead and the estimation of liability comes down significantly. If that falls the amount the trustees require NATS to fund, the now famous underlying rate, also comes down by quite a lot. In the absence of any surplus the amount the trustees will require NATS to pay is the underlying rate so if it is reduced by the cap there is an immediate effect on the pension contributions. As pension contributions are effectively paid from cash not paying those contributions is equivalent to a hard cash input.