Use of Abbreviated Callsign
My experience is no doubt the same as Will's - I take the initiative not ATC.
Typical exchange would be :-
Farnborough Radar, G-BSBA
G-BSBA, Farnborough Radar pass your message
G-BA is a PA-28 ......
After that it's abbreviated callsign all the way.
The only exception is if (e.g.) G-BRBA came on frequency and a request to use full call sign would be made by ATC.
This isn't (as Will rightly says) what CAP413 says, but it seems to universally apply and I cannot think of a single occasion when I've been pulled up. Even at Tenerife in a rented spanish 172 last week the same happened.
I too will be interested to hear the ATC point of view.
DGG