Do you work for the Network Rail press office? What if you've only got a quid?
They have change machines so it shouldn't matter what you have. The point is, you'll now need to spend not far off double.
To be honest, they should also take contactless payment cards (heck, even credit/debit cards - with a suitable fixed rate negotiated with a bank on account of the low fee).
In all seriousness, central London ones should certainly allow for Oyster to be used as the payment method.