I don't know which bank's ATM you were using, but I know that with most all of the banks I've ever been involved with, if you press "Yes" to "Make another transaction", etc. it always asks you for your PIN again, precisely to prevent these kind of mistakes. I'm not sure, but I wonder if the teller's cool reception to your honesty had something to do with this - as in you couldn't have done much with the situation anyway.
Hopefully, our banks will soon conform to this standard, but for the time being, most of the UK ATMs don't require the PIN for a follow up request.