It depends for me on where i am going. In Europe maybe about 200 euro worth. In Asia about 150 pounds worth of local + 100 USD, same in South America. USA about 200 USD. travel with 5 cards, carry 3, Visa and mastercard in Wallet, Amex in bag, others I leave where i am staying in the safe. I use cards all the time in Europe and US, in Asia i prefer cash to bargain with and they like to take the card away to process which i hate. As for the treatment by UK customs, nothing new I am afraid. i have been stopped many times and spoken to like a criminal before even opening a bag.