Unfortunately despite having CH47s & Merlins in the "reserve fleet" you can't just give them to the front line, as the reserve fleet contains all the ac undergoing essential Major servicing, accident/battle damage repair and modification programmes. Hence, they are mostly in bits...
We can't just buy CH47s off the shelf..I think we would have done if we could source any. This is probably the rationale behind the Merlin Mk3A and the Chinook Mk3-to-SH decisions.
Taking Mk3 Merlin to sea is pointless; same deck space as a CH47, doesn't fold and half the payload. Folding Italian Merlin, with uprated engines and transmissions would be a player. Boeing are unlikely to support a marinised CH47 (unless you pay them a LOT of money)- the market is too small as the USMC have chosen a MV-22/CH-53F mix.
WEBF, yes JHC should have different size aircraft for different roles. The SK4/Puma replacement will possibly be the same airframe to fulfill the medium-sized helo roles; the FLynx is the small Colonel's taxi - the only (slightly hackneyed) snag is the CH47/Merlin issue.
But as NaB points out, the CH47 is far from optimised for the LitM role. But, in calm waters and as part of a TAG, it will always be there as it does things that other JHC assets simply can't.