"Where the proposed ATS Airspaces adjoin vertically, i.e. one above the other, flights at a common level would comply with the requirements of, and be given services applicable to, the less restrictive class of airspace. In applying these criteria, Class B shall therefore be considered less restrictive than Class A airspace; Class C airspace less restrictive than Class B airspace, etc".
From a pilot's point of view, the above quote from Annex 11 is what we operate by (There is no difference filed by the UK).
When in the UK, the only extra requirement is that the Authorities have "Requested" that pilots refrain from cruising at the base level and along the alignment of an Airway.
Thus, the base level of an Airway in the UK is considered Class G and if we are getting a RIS or RAS or even FIS, we do not expect any change in service as we cross the base at 90 deg.
Regards,
DFC