Payroll function AVERA can be used for several countries including GB. In PE04, display the code for your payroll pgm RPCALCG0. You should be able to find out the config tables used (or try wild card *51av* in SM30 - eg. V_T51AV_W)
I just googled for "sap hr configuration for averages in gb payroll" with a number of hits.
Try links
PY:GB New Holiday Pay Solutions In SAP HR Payroll | IProCon
Average Earning Calculation in SAP GB Payroll -
OXP and OSP Average weekly earning calculations in GB Payroll ! - Toolbox for IT Groups
Calculation of Average Weekly Earnings - Payroll Great Britain (PY-GB) - SAP Library
The New UK Holiday Pay in SAP Payroll: Your Options | Sven Ringling | LinkedIn