Well, the booking engines are all slightly different. The system is a mess.
If you use the Atos Origin system (used by the likes of East Coast) it is possible to specify a forward facing window seat with a power socket, however there is no guarantee of an actual view and you can't specify to sit away from the toilet. But the East Coast site will tell you what seat number it has reserved, this can be compared to a seating plan and you can get the system to allocate a different seat if you don't like it.
It would be nice if you could pick an individual seat, as others have suggested. Sadly it is not possible at present.
Things are a lot better now than the dark days of all sites being powered by Thetrainline with no alternatives though.