I would advise to bring to bring your own snacks. I have been to T5 Nero many times and only rarely found kosher sandwiches there, they seem to sell out quickly and then you're stuck. For some reason Bite and AMT in T3 arrivals seem better stocked and have a broader range.
Otherwise I would suggest taking a cab to Westfield London, which is a very large mall quite close to LHR (around 30 minutes) and had a great kosher (milchig) restaurant.