I'm not sure how the end of cash would make the future any more potentially dystopian than it already is. There are already dozens of ways the Government, if they choose, can monitor your day to day activities. Most of us already carry around a personal locator device the majority of the time that GCHQ, given the word, can locate in an instant.
If Braverman decides she wants to go full police state, then giving away where I nipped out for a pint of milk by paying electronically is really neither here nor there. If there comes a time when the State can legally, and comprehensively un-person you by denying you electronic services entirely, then that would necessitate already having full and unfettered control over private banking, ISP's, electronic comms and so on, so you are not going to have ready access to cash in any case.
You might get by for a wee while with what you have in your pocket, but your next salary isn't going to appear, you won't be able to pay cash for your amenities because you won't have any anyway due to them being cut off, and if they really want to know where you are, all they need to do is flash your image around. Someone will dob you in.
Like I said, the paranoia about losing cash is ridiculous, because the feared consequences would only arise in a society where the government could already completely ruin you in an instant anyway.