Melchett
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.
I'm sorry, but if CHF and the RAF SH community are both part of JHC, why should it matter who is operating the Merlins? I don't think you've answered Nicholas Howard's question beyond stating that the RAF need to operate both types to remain 'balanced', but the operator is irrelevant as long as the types remain in service with JHC, which is the organisation that needs to remain 'balanced'.
This is, of course, a separate argument to whether or not Merlin fulfils the 'medium' SH requirement currently occupied by SK4 and Puma. From a size perspective, Merlin isn't that different from SK, because the Merlin was designed to operate from the same spots at sea, so the footprint is very similar. Merlin length (rotors running) 22.8m, rotor diameter 18.6m, SK length (rr) 22.15m, rotor diameter 18.9m.
The downwash is a different issue!