"If (airways) have an altitude as bottom level then RPS with 1000' should do (the RPS is unlikely to be over 30mb off the actual QNH/QFF)."
From the AIP (ENR 1.7)
3.9 Airspace within all Control Zones (CTRs), and within and below all Terminal Control Areas (TMAs), Control Areas (CTAs) except Airways and the Daventry and Worthing Control Areas, during their notified hours of operation, do not form part of the ASR Regional Pressure Setting System.
3.10 When flying in airspace below TMAs and CTAs detailed above, pilots should use the QNH of an adjacent aerodrome when flying below the Transition Altitude. It may be assumed that for aerodromes located beneath such areas, the differences in QNH values are insignificant. When flying beneath airways whose base levels are expressed as Altitudes pilots are recommended to use the QNH of an adjacent aerodrome in order to avoid penetrating the base of Controlled Airspace.
Why invent procedures when suitable, published ones already exist? That is the way that accidents happen.