I went to https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html, clicked on southern UK, and it gave me "Total loss [%]: -18.6".

Changes in output due to:

Angle of incidence [%]: -3.08

Spectral effects [%]: 1.48

Temperature and low irradiance [%]: -3.77

I think you’ve mixed up total efficiency with efficiency loss.