In the US, at least, there are money counting machines in, well, the two bank branches I've been using for a decade. Different banks, one small, one medium of size, in "flyover country" (Joplin, MO). The tellers don't entirely trust them, but they're at minimum useful in getting a double check of amounts. And quite fast, > 10 bills/second.
"That's only 15stacks."
I read the GP as saying 15.3k bills (i.e. £306k), not £15.3k in £20 bills. In which case, that's 306 stacks.
A $1000 bundle of 20's is 50 bills. Usually they're wrapped up like that. That's only 15stacks. You'd only really have to count one of the stacks, if you didn't trust them.