3DCAM your recollection is not quite right viz
"the Whirlwind had long gone by the time of the Fastnet incident, the Wessex was the major SAR asset at that time"
The Whirlwind was still going at the time of the Fastnet incident Chivenor didn't change to Wessex until 1982 so the mix was Seaking/Whirlwind.
I was duty pilot on 72Sqn at Odiham when the call went out for helicopter support for the Fastnet incident and was briefing a crew ready to respond when as usual when something interesting came up the "wheels" appeared and decided they would provide the crew
In the end we were not needed as there were enough helicopter assets allocated.
The SAR assests for the country have to be looked at as a whole. While yes it would be nice to have the most capable helicopter at every base on the off chance of its maximum capacity been required it is perhaps not economic.
In my time it would have be daft to have a Seaking at Brawdy as well as one at Chivenor the Wessex at Chivenor was ideal for most of our callouts which were generally local holiday makers as cliff fallers/cut off by the tide/blown out to sea on inflatables or windsurfers. I was never maxed out on capacity.
When I was at Brawdy on the Seaking we did the night/ long range/big ship rescues backed up by Chivenor if required.
Another major incident I was involved in was the Air India 747 blown up west of Cork. Although we were the 1st there that night we were soon followed by 2 more Seakings from Brawdy (one OCU a/c), 2 Navy Seakings at least one Chinook and a US Jolly Green CH53.
Helicopters are very mobile assets so I wouldn't get too worked up by having a mix of types so long as both types are capable of meeting their design requirements. (The 139 being the unknown quantity)
HF