I think Sea King Mk6C is a prime contender to replace the Puma as a 'stop-gap' measure, ie until 2040. There is still about 40 airframes which could be converted relatively cheaply as has already been said the rationalisation in the support chain would be huge (60+ people in the Puma/Gazelle IPT for starters-including my wife!).
It would be nice to think the whole fleet would than be 'Carsoned' with the new blades/Indian tail rotor/new lift frames, effectively re-lifing the airframes and giving us the 140kt (vmax, 125 kt cruise), 7,500-8500lb payload, as well as up to 25% more power, but that would involve significant investment and smart acquisition only applies to shiny new kit which runs massively over budget, not commons sense.
Replacement for the Puma - Land Rover Wolf and 4-ton trucks of course.