Planespotting in itself isn’t bad or against the rules assuming it’s part of daily exercise and you’ve walked from the home, possible at SEN, LTN and LHR. Of course it’s a different matter if you’ve driven 20 miles to get there or been standing there all day. Not that there’s too much to see.
I mean, if you know a C17 is coming and when and you get there at the right time good on you for taking a few pictures. If you start mingling with others I’d have no sympathy if you got carted off by the old bill