Understood, but that doesn't quite answer why the RAF needs a balanced (whatever that means) SH force.
Because like it or not, the RAF is still the primary operator of SH in Defence. The CHF have a very good, albeit relatively small SH capability to support 3 Cdo, but by and large, SH is still an RAF function. Just as AH and LUH is still an Army function.
And by balanced I mean that to guarantee operational flexibility, you might want something other than a 100ft x 60ft ac weighing in at smidge over 10 tons. There are times when size really does matter - regardless of what CinC Home Command tells you - and there are times when small really is better. But if all you have is a CH-47, you could well be operationally limited which you wouldn't necessarily be with a balanced force of large and small SH.