Hummingfrog,
I agree that the A139 is a step up from the Whirlwind, ie a single engine helicopter with limited range and speed, basically day VFR for SAR. However, I think there were capacity problems, for example during the Fastnet Race disaster where the Sea Kings ran out of capacity to rescue all those that needed help and the Whirlwinds lacked the range to do much. But nostalgia isn't what it used to be!
A mixed fleet might be sensible providing the types are mutually supporting, for example a S61/S92 at Lee on Solent and a daylight only A139 at Portland. To lump all your eggs into one basket by having the only SAR coverage between Wattisham and Culdrose, responsible for one of the busiest shipping lanes in the world, based on small, as yet unproven, helicopters with possibly a limited payload is a surprising
and some might save brave choice. As Jknife points out, with all the kit a modern SAR helicopter has to carry there will not be much space or payload left in the A139.