I think you're overcomplicating. The difference between QFE and QNH for a point at a particular elevation depends only on the standard atmosphere. That's because QNH is a hypothetical construct that makes an altimeter (which uses the ISA) read the elevation at that point. The number of hPa units you have to wind on to the subscale to change from 0 to the elevation does not depend on temperature. The diagram pictured above is correct. 28 ft per hPa around sea level is close enough.
At EGBJ, while the aerodrome has an elevation of 101 ft, the runways have threshold elevations between 73 and 89 ft. Hence 3 hPa difference between QNH and threshold QFE.
Last edited by bookworm; 7th Feb 2020 at 14:38.