Thanks sweetypeabee,
I'd looked at that page, but hadn't noticed the distinction between special delivery and record mail. From what I can figure, sending it via royal mail special delivery will cost around 11 pounds, while the courier costs 26. I think I know what route I'm taking.
BTW, just to vent randomly, when the UK Passport agency sent back my US passport, they did it in a regular second class delivery envelope. Anyone could have easily felt that it was a passport. Yikes!