depends on the runway direction but if departing on 09 R the Esso petrol staion on southern perimeter road between cargo and T5 is good. Otherwise Northside where the HAL visitor centre is has a spotting area good for landing or take off on 28r
Final area is of the A30 (you have to use Bedfont lane which parallels it and look for Myrtle Ave or Caines |Lane by the recreation ground-good for 28L landers or 9R departures
good luck