See CAA IN 2015/098
http://publicapps.caa.co.uk/docs/33/InformationNotice2015098.pdf
It just means that if I had to use the radio in anger I would have a bit of official looking paper to muddy the waters of any investigation.
However as its use is likely to have resulted from a total electrical failure in IMC and the commander in those circumstances can declare an Emegency and break about any rule he likes on the grounds of protecting life I don’t see the approval will do anything practical, it will only support your case as a responsible pilot for having the radio avalable.
Update:
These radios now claim to have UK CAA approval. What this actually means, apart from it being a marketing exercise, is not clear 
The document is a self declaration only.
Tomjnx, I agree that the theoretical range is in the order of 55 miles that is why I tested it out to 50 miles, however from the strength and quality of signal it did suggest that much greater range would be available, this of course would require an increase of altitude.
A relay would however have a significant insertion loss.
A good quality RF relay doesn’t have more insertion loss than your “patch cable”.
Teledyne specs around 0.3dB insertion loss below 1GHz.
We don’t even use RF relays, just standard SMD relays and they are definitely good enough below 1GHz.
I wonder how they sensed that the handheld radio was connected.
Probably not at all, they probably just “sensed” whether the panel mount radio has supply voltage. (so you could pull the CB and get the handheld radio connected to the antenna)
it would probably have worked at a greater distance but I only tested out to 50 miles.
Unlikely, as the LOS (line of sight) distance over flat ground from 2000’ is around 55 miles. Unless you’re having Es (sporadic E) conditions.
I wonder how they sensed that the handheld radio was connected.
There are obscure ways of doing it. One I can think of is to feed a bit of current out of the connector and hope that the handheld radio has a DC path to ground.
There is a “Magic T” device in the microwave world but that won’t work on VHF, in practical dimensions.
The King description was a "mechanical relay’, it is a small box with two coax connectors one in from the radio fitted in the aircraft and the other out to the antenna. It is intended to be mounted on the back of the instrument panel by drilling three holes, two for the mounting screws and one for the small plug that is connected to the handheld. When the plug from the handheld is inserted it disconnects the aircraft radio and connects the handheld to the aircraft antenna.
I have had no problems with the thing #2 radio works as well as #1 that had not got it fitted and when I tested it the handheld demonstrated a considerable increase in range over the antenna that was supplied with it.
The whole installation is very neat with only the two mounting screws and one hole visible and is idiot proof.
At the time King were selling a mechanical relay that enabled the handheld to be plugged into the aircraft VHF antena, this increased the range to 50 miles at 2000 ft, it would probably have worked at a greater distance but I only tested out to 50 miles.
A relay would however have a significant insertion loss. How would it be operated? With a switch?
I have a cable loop in my panel, behind the yoke
and can connect the handheld into that. The range is then as much as anybody can use. The insertion loss of that is just that of the two coax connectors. I used two different connectors so that one cannot plug the handheld radio into the wrong hole and shove a load of RF up the back of the aircraft radio
For emergency it’s ok but I can’t see any another usefull function
For a pilot, sure – why would you want a handheld in a plane which has a built-in radio? It’s for emergency only.
