I think most kosher restos in Golders Green/Hendon/Edgware would only deliver to the surrounding area, I hightly doubt they'd deliver all the way to Heathrow. It won't be cost effective for them just to deliver a few bagel sandwiches, unless you're ordering a banquet platter for 8.
Logistically, Isola Bella in Westfield would be the closest, I like their food but they're almost consistent in inconsistent and slow services. I won't count on them for serving (never mind delivering) foods on time.
3hour transfer really isn't enough to go to the landside/travel/get back in time for the connecting flight, UNLESS you've got an EU passport or Iris AND avoiding rush-hour traffic.
If you're really really desperate, go to the landside, find a newsagent or M&S Food, and buy a bag of Walkers crisp (ready salted and salt & vinegar are on LBD list) or an apple.