I'm not a plane spotter but have often seen people parked up at the end of the runways at Heathrow, I am not sure if it is officially allowed though. First thing to work out is which runway and direction they are using for landings, I think it might be published somewhere but you can always look on "flightradar" or similar. You probably won't know until the day itself, maybe you can guess based on forecast wind direction.
If you are thinking about watching from inside the airport then it depends which terminal you are in. Most have panoramic windows somewhere, but as above depends which runways are active. Also, you can see quite a bit from some of the multi-stories.
