She said: "I haven't been in a taxi for quite a long time but I used to spend many an hour chatting away to taxi drivers and the wonderful thing about taxi drivers is that they tell you exactly what's happening - there's nothing that you can't find out.
"Actually, one once said to me, I remember when I was in the back of a cab, I don't think he realised who I was - it was long before I was married - and asked me what I thought about myself."
She added: "He was awfully charming and I gave him a huge tip."
The charity began in 1928 and the Duchess of Cornwall has been their patron for many years.