As most of your flights are to decent-sized airports, it's hard to tell what your flight path (on landing) is going to be, so hard to advise on the best place to sit...
However, what you can watch out for:
To/from LGW: As swanhunter points out, you can get some lovely views of the Southern English coast. Around Kent and Sussex in particular, the view of the cliffs that you can get (if the weather's good - hardly guaranteed in this part of the world) can be lovely.
To/from AMS: You can get some lovely views here. If the tulip-growing season is still on (I can't remember), the sight of the strips of brightly-coloured fields is utterly bizarre. Otherwise, on crossing from the North Sea, you get a view of the dunes along the Belgian and Dutch coastline, and the endless canals that make up the Netherlands.
TLS-MAD: You may fly over the Pyrenees, or the Northern Spanish coast. Good views for both.
To BCN: If you fly along the coast to reach the airport, the views of the city (particularly at night) are stunning. I can't tell which way you'll be flying in though...