I still use bits of cash for pretty much the same reasons as BlackTrousers.
When your formative years were made up of shopkeepers huffing while people paid by cheque, or wanted to pay by card for less than about a tenner, it's hard to get out of the habit.
Also, there's the charges. If the charges wipe out the profit of a small transaction for small trader, I feel guilty about that. There's many honest ways of using cash - paying employees and suppliers, and paying personal expenses. Just because someone works in cash doesn't necessarily mean that they are on the fiddle.
Plus a lot of parking meters charge extra for using a card. Parking is expensive enough, without voluntarily paying more for it. It might only be 25 p a time, but it adds up. It also illustrates that total reliance on cards is probably making it more expensive for all of us. If shops set their prices based on someone buying a mars bar with a card, which could easily add 20 or 30 pence to the cost, everyone will pay the extra cost, however they pay.
And there's stll plenty of parking meters and small traders who don't take cards, or don't for small amounts, so it might work to only use cards if you never leave the city, but if you go out into small towns, villages and coastal areas, cash is often still essential.
I also hate the group restaurant situation where some people pay by card, because it's a lot harder to work out who's paid what - with cash, you can see from the pile of money on the table, that everyone's paid up. If you have a line of people paying by card, it takes forever, and then when you get to the last person, either there's a load of extras that need paying for that other people have forgotten about, or they'll find that everyone else has already paid the bill and they'll just pay the little or nothing that's left.