London and possibly Paris by Eurostar. Tips and advice?

I am taking a trip soon to London and will possibly be adding a couple days in Paris. It's been a hot minute (few years) since visiting the UK and would like to solicit some timely advice and tips. One thing that comes to mind: How important is it to carry cash? Would I need to pay London and Paris street vendors or any shops with cash or will a card with no exchange fees suffice? While we will of course be visiting some tourist attractions like museums, most of our time will be taken up with book stores, record shops, food and coffee shops.