I'm no expert, but my guess is it's unlikely to be worrisome. Apart from anything else, I wouldn't imagine it would be your GP who would be talking to you if there was a possibility of malignancy - it would probably be an endocrinologist - and it wouldn't be a routine appointment. That usually means, "there's nothing to worry about, but we just want to be sure you have all the information and have an opportunity to ask any questions you need to."

I am slightly surprised you haven't been given an appointment with the endocrinologist anyway. Even if everything is benign, I would expect them to give you the results. "Book a routine appointment with your GP" really sounds to me like "there is nothing to worry about, but obviously, somebody has to let you know that and you may have questions, so contact your GP and he'll give you the necessary information."

The "sinister pathology" does seem out of keeping with the rest but it doesn't sound to me as if they are worried.

This is just coming from my own experience of thyroid cancer when actually today, I had to go to the medical team just to receive the information that a CT scan showed absolutely no sign of recurrence. Even that was neither just sent to my GP nor was it a routine appointment. I had an appointment arranged for me with the endocrinologists' team three days after the CT scan.