Having issued CPL licences etc when I was at the UK CAA, I can confirm signatures were not required when I was there. In all honesty the staff there just look for 3 flights in 1 calendar day where a 300nm flight 'could' have taken place (ie A to B to C to A (if these were all within 10 nm of each other and you logged 4 hrs flying, I don't even think they'd catch it)). There is no tape measure, wind calculations or anything remotely accurate done in the licensing dept when it comes to cross checking the CPL XC flight. No signatures or QXC certificate required as it was for a PPL. Strange that you need it for one but not the other, but that was the case. Doubt it changed!