Here's a thought...
Go to a vegan restaurant.
Order a big meal.
Offer to pay in new fivers.
As new fivers are legal tender, if they don't accept them you do not have to offer an alternative and can just walk out. Legal tender MUST be accepted in settlement of a debt (which is why you have to accept a £20 quid note, but not 2000 pennies; one penny is legal tender, over 20 is not. https://en.wikipedia.org/wiki/Legal_tender#United_Kingdom )