"The IMF's resources mainly come from the money that countries pay as their capital subscription (quotas) when they become members. Each member of the IMF is assigned a quota, based broadly on its relative position in the world economy. Countries can then borrow from this pool when they fall into financial difficulty."
So the Bank of England was just using UK funds.
https://www.imf.org/en/About#:~:text=The IMF's resources mainly come,they fall into financial difficulty.