The aforementioned packing experimentation has been underway. I got all the important stuff laid out, and tried to fit it into the canvas sword bag. It was going a bit too easily at first, which was explained when I realised the longsword was sticking out through a hole in the seam. Well, it's a bit well-worn, this bag, having been stuffed to near-bursting several times. More rearrangement, and it was full. With some things still to go in. I could probably squeeze everything else into hand luggage if I tried - except for the kilt. Sigh. Looking increasingly like the torpedo gets to go, then.
I tried opening the bag to unpack it, and the zipper came off in my hand. I'd noticed that seam was going, too - didn't notice it was the only thing keeping the zipper in place.
Drat. So, it'll be the torpedo.
On the plus side, if I'm reading BA's website correctly, I can take the torpedo and a checked piece of luggage. But I'll call them tomorrow to check,