The problem with crossing the base of an airway defined as an altitude in the UK is caused by the Regional Altimeter Setting Region system of operation.
One could find that if an aircraft attempts to cross at the base altitude but is using the regional QNH, the aircraft may well be within the airway if the base level of the airway is defined using a local or adjacent TMA QNH........example the former G1 weat of the London TMA.
A simple answer in todays system is to scrap the RPS system in most of the UK and designate a conveniently located airfield or met reporting station (with an ATIS) that would provide the actual QNH to be used in a certain area.
This would make the appropriate and up-to date QNH far more available........I have waited for 15 minutes on London Info for just an RPS due to R/T congestion.
Regards,
DFC