London-UK, England, Scotland Ireland

You could almost certainly struggle to spend more than £20 or €20 in cash.

Purchase you local currency when you arrive at a bank ATM. I usually buy about $150 worth.
I usually don't spend it all, so I bring it next time I visit.

I have the same £20 note I've been taking for the past few years. Traveling in the UK I have yet to spend it - virtually everywhere takes card.

If you're question was more figurative than literal (ie you're asking about budget) that's tricky as it depends where you are and what you're planning to do. For meals in London I would generally budget £10 or so for a sandwich lunch at Pret plus a drink, and £20-£30 for a hot meal and a drink in a casual restaurant or pub for dinner (i don't drink alcohol). You can spend a lot more at a nice restaurant (ie £100 a head plus) or you can spend less if you're happy with a meal deal wrap and a drink from Tesco for lunch and a meal at a Wetherspoons pub where they do deals.

I like to at least have the equivalent of $50-$100, depending on how long I'll be in each country.

While most places take credit cards now, i often run into at least one place a trip where the card reader is down and so cash is needed.

I always get the cash from my bank (Wells Fargo) before I leave. i don't want to have to worry about an ATM machine eating my card in Europe.

While most places take credit cards now, i often run into at least one place a trip where the card reader is down and so cash is needed.

It is amazing how quickly these machines start to work when you tell the proprietor you have no cash. Just a mystery of modern technology I guess.