Cash in London

For starters, please read this: https://www.ricksteves.com/travel-tips/money

The cheapest way to get cash is from a cash machine (ATM) once you arrive. Cheaper than exchanging banknotes either in your home country or in the UK.
If by "take" you mean how much cash you should get before you depart, the answer is as little as possible, just enough so you have a safety net for 24 hours in case you can't find an ATM at the airport.
Some people feel more comfortable with a few £££ in their pocket when they land, some are happy just to go to the first ATM.

As for card acceptance, Visa and Mastercard OK (credit and debit cards), others (American Express etc.) patchy acceptance. A restaurant 99% certain will accept cards. Small shops may have a minimum amount for credit cards (£10 or £20) or market stalls probably cash only.

When you arrive at Heathrow, get £/Pounds at an ATM. Easy and the best rates available. Get a couple of hundred. We were there recently for ten days and mostly used credit cards except for small purchases ( stamps, postcards, snacks, tips)
I hope you have your tickets for the Buckingham Palace tours inside the State Rooms. If not, go to the www.RoyalCollectionTrust.co.uk and book them today. You will have a timed entry. Add the Gardens for a nice experience too.

Be aware the ATM machines at Heathrow are not bank machines but belong to Travelex. I''m not sure if they give the same exchange rates/fees as bank machines.

As has been tested, posted, and re-posted many times, the Travelex machines at Heathrow give you your cash withdrawal from your bank account at the standard interbank rate. Just make sure you don't let it charge your account in your home currency.

Be aware the ATM machines at Heathrow are not bank machines but belong to Travelex. I'm not sure if they give the same exchange rates/fees as bank machines.

ATM's do not do the exchange. ATM's charge the amount you get out, in that currency.
The card issuer (Visa, Mastercard etc.) does the exchange into the currency of your card, so the rate is the same for all ATM's.
Except if you are suckered into the dreaded DCC ("Do you want to be charged in your own currency?"), in which case you can guarantee that the rate the ATM owner uses is worse than your card issuer would have given you.

I was there in June and found that a lot of cash was not needed. As long as you spent at least 5 pounds you could use your credit card if you prefer that to cash.

I do it old school and convert cash before going over. Eventually I'll use ATMs, but right now have plenty of pounds and euros just sitting around that I bought at a great rate. It's really a personal preference, but you should have some cash upon arrival for water, sandwich, or miscellaneous things. Most places throughout the country take credit cards, but there are some mom and pop establishments, some pubs and restaurants that only accept cash. As others have said, M/C and Visa are widely accepted, AmEx, not so much. Don't even think about taking Discover. Remember to tell your CC company you're traveling and ensure that all cards (credit, ATM/debit) that you may use are included in the travel notification. I'm heading over later this year and most things are already paid for, or will be paid for before going over, so I already know I won't need much cash.

Recently returned from 9 nights in London, got some cash at an airport ATM but could have used Visa or MC for everything, including vending machines.

We found cash best for Camden Market---may or may not be true of Portobello Market, but it wouldn't hurt to have some just in case.

Sure, go ahead and use your credit card like you do back home, without reading the fine print. After the monthly bill comes in, add up how much that convenience cost you. Many credit cards charge fees per transaction. Just because there is no fee back home does not mean a foreign country such as the UK won't incur a fee from your own bank. As many travel websites will tell you, VISA and MasterCard are widely accepted, some of the others not so much. And it will be up to the small merchant individually whether he can afford to offer credit through a card. I retreat to cash for all except large purchases, saving the transaction fees by taking out a large amount on an ATM/debit card and then acting like the old days.

Since no one else has mentioned it, the V&A is free entry, as are most London museums, although donations are appreciated. For everything else, do yourself a favor and buy tickets online. I had more than one experience this summer of entering ahead of 200 people waiting in line because I'd bought my ticket ahead. Even if they charge a processing fee, you will come out ahead in terms of the time you save waiting on line.

Most places will take credit card which is what we used most of the time. Bring cash for Portobello though! I usually exchange a small amount at home and then use ATMs if needed.

If using a credit card for purchases, use one with no foreign transaction fees.

FYI in Britain an ATM is called a "cashpoint."

While the Victoria & Albert is donation requested but not required for the permanent collection, the special exhibits are often pay ticket and some of the popular ones will sell out time preferred time slots ahead of time. So if you want to see Pink Floyd at the V&A, best to buy the tickets now and print them out at home. Also check availability of House of Parliament tours and probably good to book beforehand- they only run on most Saturdays and then only on weekdays when Parliament isn't in session. Globe Theatre is a walk-up ticket purchase.

Has anyone ordered foreign currency from their local bank before traveling?

I heard you can do that, but never tried it myself. I would feel more comfortable landing in Heathrow with some cash in my pocket. I've gotten left holding the bag in the past when I relied solely on plastic (account locked due to suspicious activity). Not sure what a back up plan would be if you land with empty pockets and find your bank card gets unexpectedly declined at the cash machine.

bob_m58,

I have in the past gotten some euros and pounds from Wells Fargo prior to a trip, however I haven't done that in the past 3 or 4 trips as I know that it is very easy to get money from an ATM upon arrival - also I almost always have some cash left over from previous trips. If it will set you mind at ease you should check into doing this. It is very easy - just a matter of going to the bank and making the exchange - unless you are trying to get a large amount. If you want a large amount the bank may need to order the currency.

Be sure to tell your banks and credit cards the countries you will be traveling to and the dates. That should prevent any problems with the cards being declined in Europe. I can do this for all of my cards online.

Get the RS London guide book and it has a wealth of information and will tell you al about the attractions you want to visit. I just got back from London and spent a full day at the Victoria & Albert musuem and had lunch there in their cafe. It is a free museum but they do ask for a donation but that is up to you. I bought my ticket before I went to Buckingham Palace and it is a timed ticket. They are only open for 10 weeks out of the year. Do check out the websites for all the places you want for times, reservations, prices and more. Have a great time. Portobello Market was excellent but I paid in cash when I bought something.