Pointer:
It's in connection with (military) maritime patrol flights where one often operates below FL055 without need of a clearance.
Eric T:
Is the UK 5500' written down anywhere? The problem, I guess, with 5500' would be the need to cross check with 1013/29.92 before reaching 5500' to see if one had reached FL055! (and thus remain below controlled airspace, if that was the aim!)