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.

those who have been holding English banknotes as standbys for some years should make sure they are not old paper versions, polymer (plastic) have taken over. Small shops may refuse £50 notes for small purchases.
https://www.bankofengland.co.uk/banknotes/current-banknotes

English notes are accepted in Scotland and NI but the reverse is not always true, most people in England have never seen an NI Banknote and quite a lot in the south are wary of Scottish notes.!

I would start with £20 and €20 for the entire trip. You can always stop at an ATM/cashpoint for more if you need it.

2 trips covering about 4 weeks to England and Scotland in the past couple years and we spent, maybe, a total of £20.

You will probably not need more than 100 of either the entire trip as most purchases / payment can be made with a credit card. However, we have experienced a number of instances when credit or ATM cards did not work as they should for a variety of reasons. Mainly because technology does not always work as it should. That is a simple fact of life. That is when it is good to have some cash as a back up.

With regard to getting local currency from an ATM upon landing, consider the following. You will be tired, jet lagged, sleepy, on new turf, and probably in awe of where you are. Most people (self included) don't need to be handling sharp instruments or operating an ATM under those circumstances. In addition, there is the problem of finding a "real bank" ATM, waiting in line to use it, and hoping all goes well. ATMs have been know to run our of currency at times.

The "extra" cost of getting 100 or so pounds or euros from your local bank before leaving in nothing compared to the overall cost of the trip. And it's a small price to pay to eliminate one more hassle upon arrival.

Regardless, be sure to check with your bank and or card provider to see if you need to place a travel alert on the ATM and / or debits cards. The same goes for any credit cards. Some require a travel alert, and others don't.

When I was in London in February, I kept thinking I should get some cash but never got around to it. So I spent a week there totally cashless. You don't need much if any at all in London.

In the past 4 trips to the UK post-covid, here is where I have needed to use cash:
1) At a kiosk on the street to buy a bottle of water
2) Every year at a coin-operated laundromat (they made change for my 10 GBP note)
3) At a Caffe Nero (coffee shop) when their network was down. They had a sign on the door and had to turn some people away who only had cards. Note that I could have walked another block to a different coffee shop with a working network.

For the UK, I ’d probably just get 20 GBP out of the ATM.

For the Republic of Ireland, I’d take out more in Euro because I could use those Euro on future trips to other countries. On my 2-week trip to Ireland lst year, I did use Euro a few times for small purchases but I don’t recall how much I needed. Most places took cards.