Another example of usage
"You lot" is not the only possible usage. Consider this example:
Well, I just thought... maybe... it was something to do with... you know... her lot.
(From JK Rowling's Harry Potter and the Philosopher's Stone)
In the U.S. edition of the same book, the expression "her lot" was replaced with "her crowd". Apparently, the editor did not consider this usage intuitive enough for a U.S. speaker of English.
In my opinion, "you lot" might have evolved from the original meaning of the word 'lot'. I cite dictionary entries below to show how the meaning of this word is extended in British English.
From Cambridge Advanced Learner's Dictionary:
lot plural, noun, UK, informal: a group of people.
You're an ignorant lot!
Are you lot coming to lunch?
My lot (= children and family generally) won't eat spinach.
the lot UK, informal: everything.
I made enough curry for three people and he ate the lot.
Have I got everything? Is that the lot?
I'll sell you the whole lot for only £50.
I'm sick of the lot of them.
From the Oxford Online Dictionary:
lot noun [with adjective] chiefly British:
a group of a specified kind (used in a derogatory or dismissive way):
an inefficient lot, our Council.
From Merriam-Webster's Learner's Dictionary:
informal a [count] chiefly Brit : all the members of a group of people — usually singular.
Do you know the lot [=bunch, crowd] that hang around the arcade?
That lot will never amount to anything! They're a thoroughly bad lot.
Pipe down, the (whole) lot of you. = Pipe down, you lot.
In British English, a person who is not liked is sometimes described as a bad lot.
He may be a bit wild, but he's not a bad lot once you get to know him.