I think you must be an ATCO Louby, and not a pilot. You are making an assumption that when a controller states "remain outside controlled airspace", they are 1) controlling you, and 2) they have some jurisdiction over the airspace in which you are flying or into which you are going to fly.
I regularly fly from Northern England to Brittany and the Channel Islands. Sometimes airways sometimes in the FIR. Quite often Brize, when called fo a radar service will say "Remain clear of Brize Zone, Stand By." OK, I take the point, but I will be at FL 060 and their zone goes up to 3500 ft. I might not even be flying that near to it.
Then further south, Bournemouth have a habit of barking this instruction at you "Remain outside controlled airspace.Stand by". I might well in IN controlled airspace. Or, more likely, I am going to overfly their airspace well above CA.
It is all very well suggsting that pilots say too much on the air, but the system in the UK requires so much more to be said. The controllers asking you to keep clear haven't made this instruction up themselves: I am sure they have been instructed to say it, because SOMEONE somewhere has decided that as some aircraft do inadvertently enter CA without permission, this might help to stop it.
Well it might. But it also adds more talk And this SOMEONE is coming up with all sorts of arcane and special UK R/T requirements. Years ago we were striving (I thought) for commonality with our foreign friends. Now the British keep coming up with all sorts of sometimes bizarre changes to R/T procedures in order, it is suggested, to prevent some allegedly possible confusion from arising.
I know you will argue, Kiltie. You'll say that anything that can be done that will help prevent any sort of incident whatoever should be done, etc etc. But I think it is time to call a halt to all these British R/T add-ons which often just sound plain daft.
Re. the "have him on TCAS" bit. OK, fair point. I must admit that I wasn't thinking of approach control at the time, and I can see that the response is a bit pointless under those circumstances. Sometimes, though, you just respond in a manner that's common without actually thinking about what you are saying. Maybe the controller shouldn't bother saying anything in the first place if the pilot is not expected to respond with anything other than Roger?
RB