FWIW, here's what LASORS (the "access guide" to the ANO, which is the UK interpretation of JAR-FCL) has to say about it. (Section D1.2(D) )
An applicant for a JAR-FCL CPL(A) must have completed
a minimum of 200 hours of flight time, including the
particular requirements specified in a, b and c below.
These must be flown in aeroplanes irrespective of any
credits applicable under D1.2(D) Notes below:
[...]
a. ii. 20 hours of VFR cross-country flight time as
Pilot-in-Command, including a cross-country
Flight* totalling at least 540 km (300 nm) in
the course of which full-stop landings at two
aerodromes different from the aerodromes of
departure shall be made.
[...]
* The cross-country flight should be regarded as a single
planned exercise including landings at two intermediate
aerodromes and completed during the course of a single
day. Flights completed over the course of more than one
day will not normally be acceptable towards licence issue.
Should an applicant claim that there were mitigating
circumstances that prevented the flight from being
completed as originally planned, the applicant must send
in a written submission to PLD explaining what happened
together with any relevant supporting documentation/
information for consideration.
Obviously for PPL the flight has to be solo since you're not a license holder yet so there's no other way to be Pilot-In-Command. But for CPL nowhere does it specify that it has to be solo. So if you have a PPL you're legally allowed to bring passengers on the x-country.
What LASORS does not make completely clear though is whether a long-distance flight with one refueling stop (A-B-C, where the distance A-C is more than 150 or 300 miles as appropriate) is accepted instead of the standard A-B-C-A flight. Also, the CAA states that the intention is to make the flight a single planned exercise, performed over the course of a single day. But they do allow for e.g. weather contingencies.