Thanks for the replies folks. I still have not found anything that categorically stops one doing this. The reason for considering this is partly that I still need the P1/solo nav time for the PPL and the qualifying cross country. Rather than embark on some random cross country flight to build hours and experience we were discussing doing the longer flight to simultaneously take care of all of the required experience/hours/P1 time and cross country for the PPL as well as the CPL.
I'm sure I probably will do a 300 mile flight in the 100 hours, but I thought it would be nice to have this requirement 'ticked off' early!
Cheers
B