I have indeed checked it, and as far as I can tell we are showing the airway exactly as the AIP defines it. I assume you're referring to the turning point at 561400N 0033819W where the base level changes from FL55 to FL85. There is no nonstandard base level change path at that point.
You state the airway hasn't changed for 20 years but please bear in mind airways are changed every 28 days, and this one has definitely been touched by those changes in recent months.
Possibly the author of the 500k chart you are comparing with assumed the airway base level change would be aligned with the northern boundary of the Scottish TMA (which would make sense) but there is no suggestion of that in the AIP.