Nurse,
SABR heavy was going to be a marinised folding-head CH47 (with fat tanks/radar etc etc...). -53 was looked at but -53E was out of production (and ruinously expensive to run) and -53X was too far in the future. The real pi**er is that SABR CH47 would have been arriving next year if the SABR budget hadn't been raped by £1.5Bn......
RAF Merlins don't fold - the capability was removed in an effort to boost its' lift capability (no sniggering now....). Therefore, to marinise Merlin you'd need to buy back that weight just to stay even. Happily better engines/xmsns do exist thanks to VH-71, so any CHF Merlins would need these. I agree that the Italian marinised Merlin is a good option for CHF, as long as the booties still have access to CH47 for the really heavy stuff.
The Aussies, Dutch and UAE have all ordered CH47F; the Canadians are not yet on contract. Even if we were to order now we wouldn't see an ac for 3 years and, as MM4 points out, they would be yet another fleet within fleets - unless we decided to recapitalise the whole fleet on a -47F basis. The Mk4 Chinook is not the MLU, it is a palliative measure to address issues with certain kit at the moment- though it could provide the basis for the MLU. The MLU is not planned to start for a few years yet.
We undeniably need a Puma sized ac for certain land-centric tasks, though, hopefully, any future medium buy will have some littoral capability.