Just to follow up on this thread that I started before my trip:
I ended up taking a box of dental floss to cut my thread, like someone here suggested, because I couldn't find my nail clippers. I had no problem at all with the needles (and a few pins) at the airport in Ottawa. They were in a ziplock with my hand-sewing project that was packed in my carry-on.
On the way back, I forgot that I had stowed my little embroidery scissors (which I had packed in my hold luggage on the way there) in the ziplock with the rest of my sewing stuff in my carry-on, but it still passed through security at Heathrow airport in London with no problems! I was very surprised afterwards when I realised I had left them there, but I guess they were small enough that the guy running the scanning machine either missed them or didn't think they were a threat. They did make us take off our shoes, though (which they didn't in Ottawa on the way out), so they were being very thorough.
Jan