Actually CM's
EGLK N0120F040 DCT SAM/N0120VFR DCT MP
is not strictly right. A change of flight rules is supposed to be indicated by the letters VFR or IFR on their own:
EGLK N0120F040
DCT SAM VFR DCT MP
is all that's required for a Y FPL.
For VFR to IFR (Z), the level and speed is required
EGLK N0120VFR
DCT MP DCT SAM/N0120A030 IFR DCT CPT
For a VFR FPL it hardly matters.
To answer PhilD's original question, so long as whatever ATC you're working understands what flight rules you're following, I don't think it matters.