First of all, pretty much all the kosher eateries are in NW London, about 45 mins from LHR.
Milky restaurants:
Novellino (Golders Green Road) - definitely worth a visit
Milk & Honey (Golders Green Road) - simpler than Novellino
Isola Bella (Hendon, top of Bell Lane) - huge portions, not as sophisticated as Novellino, but better than Milk & Honey
Meaty restaurants (glatt):
The White House (Hendon, top of Bell Lane) - mainly grills
Kaifeng (Hendon, Church Road) - Chinese
La Fiesta (Golders Green Road) - grills. Very good, but closed at the moment due to a fire.