UK AIP ENR 1.7
Altimeter Setting Procedures
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 belowTMAs 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 the QNH values are insignificant. When flying beneath Airways whose base levels are expressed as Altitudes pilots are recommended to use the QNH of adjacent aerodrome in order to avoid penetrating the base of Controlled Airspace.