Well, to be fair, I didn't even know who Derek even was until I read TFW. So therefore, before reading Baggins's complaint about it, I was none the wiser.
My theory is, and you're all free to correct me as I have not recapped myself on TFW in a long time, is that the Derek from TSL and the Derek that wrote the Guidebook to the Land of the Green Isles are not the same person, but they are related possibly father and son or grandfather and grandson, with the younger Derek being named after the older. It would explain why TSL's Derek is not as chummy with the Royal Family as he's supposed to be, at least. And something else too, but I can't remember what it was from last night.