Yes, we do have a Queen - and our banknotes display the Head of State on the front of the notes.
The back of the notes has become the preserve for famous Britons. There are many famous female britons throughout our history, and they have been under-represented.
Given that women represent 50% of the population and that there are continuing issues over their representation in boardrooms, in senior administrative roles, in parliament - surely it should not be that big a deal to find room for 1 or 2 famous female britons on the reverse of our notes?