A couple of data points for you:
Nov 2023 LHR-FRA (o/w) 21,943 seats 133 flights (av 165 seats/flight)
Nov 2013 26,583 seats and 195 flights (av 136 seats/flight)
Nov 2023 LCY-FRA (o/w) 8,624 seats 88 flights (av 98 seats/flight)
Nov 2013 LCY-FRA 6,308 seats 83 flights (av 76 seats/flight).
I can't dig before 2013