Good judgment is more important than strict adherence to CAP413
I don't agree. The CAA/NATS are pushing for better RT standards. All commercial pilots have already received a glossy CAA booklet which stresses this and gives many of the CAP413 RT calls.
"Er, Tower .. "
"Er, G-CD .. "
Again, CAP413 1.2.1.g - Transmitting Technique - Avoid using hesitation sounds such as "er".
The simplest thing to do is answer them in the same was as they address you.
"PPRUNE Control, G-ABCD"
"G-CD pass your message"
"G-CD is a ...."