Even if the Commandos do not get a dedicated Attack Helicopter squadron, I do think that they do need a number of lighter utility helicopters that can carry up to eight men and all their equipment, and can operate in all weathers, day and night and in hostile conditions. In fact a similar platform could be also used by the SF and elements of 16 AA Brigade. I would say this would be a category that any replacement for the Puma could fit into, and ideally being marinized, it could be deployed on Naval vessels with any worry regards damage form conditions at sea. These aircraft would be used by the AAC and FAA rather than the RAF who would consolidate on the Chinook.
As for replacing the Merlin HC4/4a, whilst being able to be housed and operate form current and planned Escorts would be useful, it should not be a compulsory requirement for the programme. The T-26 has a flight deck that can handle a Chinook so any Merlin successor could follow its lead and lilly pad of these vessels from locations further back form the target. The most likely limiting factor in the criteria for this platform will be to able to operate form the Carriers and fitting in their hanger. Few planned or current platforms will fail to do this, even the SB-1 with its contra rotating blades.
In fact the development of this platform as well as its Bell competitor fall very nicely into the timeframe the MoD is looking at. Both would provide A jump in capability over existing platforms and as the US Army programme is also looking to replace the AH-64E with a variant or relative of the transport platform, serious consideration should be given. It would take a change in doctrine, moving troops to, from and around the battlefield in smaller units, especially in the initial stages of an operation, but there will be the RAF's Chinooks available for heavy lifting, especially if these are modified for operations at sea properly. These new platforms would open new option for conducting air assault operations, being able to fly further and faster then either the Merlin or Puma. It would also mean the RAF can get back into its more traditional role of providing support helicopters rather than lifting the assault troops, though that option would still exist. If say the SB-1 was chosen than its attack variant would be an ideal choice to replace the Apache. The fly in the ointment is that this is a US Army Programme not the USMC. But if we are lucky the latter may decide that variants of these platforms modified for operations at sea would be suitable to replace some or all their recently introduced Attack Rotorcraft, given the range and speed benefits. If this were to happen, we could join the programme at this point getting the capabilities we really need.
Any replacement helicopter for the Merlin HC and Puma needs to be able to operate form ships over long periods of time so that it can be a common platform for the AAC and FAA, as does any successor to the Apache. There could be room for it to replace the Wildcat as well in both its land and seaborne roles.
Finally the Merlin HM is and will be the ideal platform for ASW duties for decades to come, possibly staying in service longer then its predecessor the Sea King. It is the king of ASW helicopters, unmatched in capability. One interesting option for it would be the fitting of a AAR probe to extend its range, either being supported by land based AAR platforms like the KC-130J or A400 or, from a COD/AAR platform if the MoD ever decides one might be useful.