In my brief time recently considering trading the Glee for a new bike (I came to my senses), I learnt that dealers don't give a monkeys what you've added to the bike. As far as they're concerned, the book says £X,XXX so that's what it's worth... you could have plated it in gold and it'd still be worth that much. Possibly a few extras that they see as making it easier to sell on like a top box may grease the wheels of a deal slightly, but in my experience is doesn't seem to add a massive amount to the money they'll give you for it.
People, on the other hand, do sometimes value the extras you've added to a bike if they're things they were planning to add anyway. So my theory is that if you've kept your bike as stock it may be worth part exchanging it, but if you've added lots of good stuff, you're better off with a private sale.
This isn't based on a vast amount of experience though... but it's the impression I get.