Does that key give a backslash on its own (US), or a hash (UK)?
The keyboard is a US one, and the US driver should give "\" shifting to "|" as shown on the keycap. The UK driver in the "international with dead keys" variant should give "#" shifting to dead tilde, with "|" on the third level.
I don't know anything shifting to "~" with "|" on the third level. What output do you get from
If you can't get the right driver, you can edit one. They live in /usr/share/X11/kxb/symbols/ and the key you're having trouble with is called <BKSL>.