You should only use the abbreviated callsign if the controller (or FISO etc) you're speaking to does so first... eg if you call "Airfield Rader, G-ABCD, request flight information service", and they reply "G-CD, pass your message", then you're ok to refer to yourself as G-CD from that point onwards - otherwise you must stick to the full G-ABCD.
As for other callsigns - not sure of the rules, but my flying club uses "special" callsigns... eg when under instruction, I might be "Thanet 33", but flying the same aircraft solo, I'd be "Thanet CD". This system seems to work reasonably well in the local area although a certain popular airfield near Ashford doesn't recognise the "Thanet" callsigns, so one has to revert to the full G-ABCD - just one more thing to remember!
cbl.