The reference you need is LASORS, Section A, Appendix F, Part B. Download LASORS from
http://www.caa.co.uk/docs/33/LASORS_07.pdf and scroll to page 87, where it says:
'Cross-country flight' means any flight during the course of which the aircraft is more than 3 nautical miles from the aerodrome of departure.
Since Appendix F of Section A is simply lifted from Schedule 8 of the ANO, I assume you could find the same text there (which would make it legally enforcable), but I haven't looked.
FFF
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