Traveling from AUS- BLR on Delta/Virgin Atlantic with expired visa stamp but a valid i797 via Heathrow airport.

Hi- I’m traveling from AUS to BLR on Delta/Virgin Atlantic via Heathrow. I have a valid i797 but my visa stamp itself is expired. I checked at the airport with delta airlines, they said they only need the A-number that’s on i797 if visa stamp is expired. Is this correct? Will I be allowed to board? Anyone travelled recently with similar situation can provide inputs?