Who wants a mustard bath?
I had a look and can't see that Colman's still does mustard specifically for the bath, but there are a number of providers that do, and swear by its power to relieve aches and pains and 'flu symptoms. I even saw a discussion about whether it helped Covid 19 sufferers (they concluded it didn't).
You're supposed to put it in a very hot bath and marinate yourself for 10 minutes. Personally, I wouldn't recommend it.