It is illegal to deface British banknotes, but you can destroy them.
Under the Currency & Banknotes Act 1928 it is illegal to deface banknotes by printing, writing or impressing upon them words, letters or figures, however you can rip them and destroy them Piers Morgan did on This Morning when testing the durability of the new five pound notes by ripping them in half and sparked a twitter outrage with thousands calling for his arrest for high treason against the Queen, sadly, in vain.
