You can literally step off your train and board the Skylink. This is a comfortable and fast connecting service that will take you from the train station into the main terminal.
Some of the big ones I have seen there:
Emirates A330, just once in a while operated in a 777.
Access to the observation deck it's worth 1 pound, however there is a lot of entertainment there. CCTV covering all the terminals, that you can operate yourself.
As to the spotting areas, it will take may be 10 minutes walk to get from the main terminal to the long stay parking. If weather is not favorable you can shelter in one of the bus-stops though.
If you have more time, you can walk around since there are other interesting places to go in the airport, however you could need the whole day to do it.
LHR, BHX, EDI, BKK, USM, CNX, PHU, GRU,PEI, BOG, CTG, CPH, AMS, DOH, DXB, FRA, MAD and always PEI