Ripping money is not illegal in the UK.
However Defacing Money in the UK as well as the USA is illegal.
The Currency and Bank Notes Act 1928 is an Act of the Parliament of the United Kingdom relating to banknotes.
Among other things, it makes it a criminal offence to deface a banknote (but not to destroy one).
It is not illegal to deliberately destroy a banknote.
However, under the Currency and Banknotes Act 1928, it is an offence to deface a banknote by printing, stamping or writing on it.
It is unlawful to deface, mutilate or render unusable any US coin or currency.
I have literally never heard of anyone being charged or prosecuted for it, however.
Technically yes, its illegal to deface US currency in any way.
But ripping a bill in half isn't likely to get you in trouble.