I've been trying to find out a few better answers but not quite finding the final answer.
In those parts of the Worthing CTA where the airway bases are FL65 and FL75, London ATC have procedures when the QNH falls below certain values, the lowest available flight level rises. Basically, under those segments, the transition altitude of 6000' is always outside CAS and always available, the airway loses levels to provide separation. I cannot find any entry at the moment that relates to the FL55 segment but will post back when I do.