The exact opposite happened to me last week, I went into Volcano cafe near Crystal Palace and ordered some shortbread and a cup of coffee, and was told they don't accept cash, at all, ever. I was gobsmacked, never heard of anything like it, well I have actually, exactly the same happened to me last time I tried to get on a London bus, again could'nt pay in actual cash, I felt like an idiot abroad.
And annoyingly I did'nt have a card/oyster on me. Volcano must be well up the creek today.