VMC Climb & Descent. MATS Pt 1 Section 1, Chapter 3 para 5.5 refers.
To avoid excessive delays to traffic
when radar is not available (my italics), controllers may authorise an aircraft to climb or descend in VMC provided:
6 provisos which can be found
here. However, the important bits are - essential traffic given, the pilot agrees to provide own separation, in daylight hours, in VMC, in Class D,E,F and G airspace below FL 100.
Having said that I recall a day at Preston Centre (that dates me) when the controller on a busy Pole Hill Sector (no radar) said words to the effect of "Can you see the 707 contrailling ahead of you?" Answer in the affirmative. "Roger, formate line astern and climb through". Ah, those were the days. This all said, by the way, with a pipe firmly clamped between the teeth.
Sorry, drifted off the thread.