Even when assigned an IFR clearance, simply reading back the transponder code is sufficient, and I doubt that even that is legally required.
I think you will find (and I don't have an AIP handy so can't quote verbatim) that receiving an IFR departure clearance you can just read back the transponder code
and your call sign but there are a few caveats:
a) your clearance must include a SID
b)you must be departing from a controlled airport
c)you must include any ammendments to the SID i.e altitude
d) your routing must be what was filed otherwise you will be required to readback the clearance in its entirety. The controller in that case normally advises that he/she will require the full readback prior to giving it to you initially.