I’m not finding anything on Google offhand that goes higher than that, except for one minor reference to a 16 tog. It’s possible it’s just a typo on the web page.
And a lot of the 13.5s are already billed as “extra warm for winter use”.
Are the ones you have stuffed with man-made fibers or goose down? A goose down 13.5 or 15 might feel warmer than a polyester or dacron 13.5 or 15.
Can I suggest an electric blanket? They rock. I had to live in a drafty old house in Michigan for two years, and it was the only way I survived the winters.
[privately wondering what frigid hell Flapcats is inhabiting]