Maybe. You could try contacting one of the kosher restaurants first. Most of them do home deliveries. You could clear passport control and go landside, and arrange to collect from them there. Best would be to meet them outside Departures, so they would not even need to pay for the parking. That way you would only pay the delivery charge and not a two way expensive taxi fare (and I think they charge less for takeaway as well).

If you arrange it in advance, you could set a time about an hour after landing, which should be plenty.

Try Isola Bella (milky). Another one would be Hendon Bagel Bakery, who do very tasty bagel/sandwich platters. If you prefer meaty there are a few places, depending on whether you want glatt or regular kosher. A good place to start would be www.kosher.org.uk (London Beth Din Kashrus Division).