How do we respond to tag question after urgings starting with 'Let's'?
I agree with Edwin Ashworth's comment although I'm not sure about the necessity for a semicolon.
My answer
A: "Let's go, shall we?"
B: "Yes let's."
If pushed I might insert a comma and say, "Yes, let's" but personally I find that unnecessary.
My U.S. answer:
A: "Let's go, shall we?"
B: "Yes, let's!"
It doesn't sound old-fashioned or jarring to me, as it did to Nicole, but maybe it's a generational thing.