I’m from California and studying in England for the next couple of months. In class today someone asked me if he could borrow my rubber. I look at him for a second seriously considering giving him a condom from my backpack (He might need it). But it turns out that they call pencil erasers rubbers over here.
A good opener for back in the states is to ask a girl for a rubber, let her squirm for a few seconds and then say “A pencil rubber. Um, an eraser!” She’ll be glad when you relieve the tension and explain the silly miscommunication. This opener might require some bullshit back story about studying in London or being raised by Brits or something.

it would require an accent, man. anything less, and it smells like wierd BS.
and when you explained it to her, you couldn't say "an eraser" because most likely you wouldnt know that word, if you did you would have used it as brits are aware of the american venacular for condom.

so it would have been

You: 'Ave ya got a rubber on ya? can I see it just quick?
Her: WHaaa?
You: You know, a rubber! Tha' bit ta wipe out marks an' such..

or you could just say

you: can i borrow your rubber?
her: (looks at you funny)
you: (if she is holding a pencil) the thing on the bottom of the thing you are holding
her: oh
you: what did you thinking i was talking about *smile*?

basically you are playing it like she is the one with the dirty mind.