I think anything above 4mpkwh is good. I tend to average around 4.4mpkwh, but that’s partially due to manually controlling when i use the petrol engine, so the electric motor always does the juicy, creamy bits that are good for economy.
Below 3.3mpkwh i’d start to get a bit miffed for longer journeys, regardless of weather.
Your dropoff won’t be as bad (still VERY noticeable, mind) as many other EVs due to the standard equipment heatpump.
Prior: SEAT Ateca Xcellence Lux 1.5 TSI DSG MY19, VW Golf GTE PHEV DSG MY23
Current: Hyundai Ioniq 6 Ultimate
Next: we'll see what's available in 2028.