(i) There are plenty of places where it gets down to "only" -5C or -10C where it would still be practicable; you're giving worse case scenarios but those don't apply everywhere. We can do what we can where we can. (ii) Bring it in your pannier or backpack (buying wrinkle-free helps as well).

* https://www.londoncyclist.co.uk/how-to-carry-a-suit-on-your-...