I used a National Railcard discounted Oyster card with Zones 1-2 Travelcard loaded. My journey was Willesden Green – West Hampstead – Hackney Central – Chingford off-peak.
The fare charged was £1.25. However a standalone fare for a Zones 3-5 journey on the same scale (Clapton – Chingford) is £1.45.
There are fewer scales when it comes to extension fares, so you’ve been charged at the TfL-LU rate. You’d only be charged more if the journey was on the NR or NR-T scales.
Sorry for the delay replying. I don’t know for certain, but I’d expect in this case that the additional premium would apply. If you make the journey, perhas you could let us know.
I have tried Gatwick Airport to Willesden Green and the fare charged was expected – the same as Gatwick – Balham (Zone 3) which was £4.80 off-peak with National Railcard discount and Zones 1-2 Travelcard loaded.
I’ll fly from Heathrow next time and will report my finding if I use my Oyster card for a through journey.
My return journey after landing at Heathrow T4 was off-peak but I decided against travelling all the way by rail back to Willesden Green (with my Zones 1-2 Travelcard and National Railcard discount), as I was pulling heavy luggage and wanted to take a bus from Acton Main Line to get home. I paid £4.60 for the “privilege” as the journey was in Zones 6-3, totally outside my Travelcard coverage.