Can anyone tell me some of their favorite things to do in London? I'll be there for four days at end of September into October. One day we'll be going to an NFL game but the other three we're looking for things to do. I really want to do the Harry Potter tour but not sure if that will happen as I doubt the other people we're going with will want to and I would like to stay together as much as possible but we'll see.
Would it be best to bring cash and have it exchanged or use a no foreign transaction fee credit card or a little of both?
I can't help much on the things to do. My plans got a little derailed, but the Tower of London was a good few hours. If you have interest in historical arms or the crown jewels it was interesting. Also saw a changing of the guard outside the jewel tower. I chose to walk through the Superbloom exhibit in the moat, but they have public viewing spots if you don't have enough interest for the separate charge to walk through it.
As far as cash, I personally bring a debit card and withdraw from an ATM. Has the best exchange rate and I don't have to wait in line at the exchange counters. I have both Fidelity and Schwab checking accounts that have no FTF and reimburse ATM fees, so I only take out a small amount of cash as backup. If I need more, I find the nearest ATM. Everything else, I put on a no FTF credit card.
Also, just a word of caution about cash, don't hang on to it for 15 years!

The last time one of us was in London, it was my husband and he had to leave under unexpected circumstances. London had a terrorist threat the day before he was supposed to leave that shut down the airports. Fortunately, they reopened for his flight the next day, but it was chaos as they were advised to get to the airport so many hours ahead of the flight - which for him put it around 3 or 4 am - but they didn't open the airport so it was just hundreds of people loitering outside the terminals for hours. When they did open it, it was just a crush of people and you weren't allowed to take any bags as carry-on so lots of repacking at the check-in desks. Last thing on his mind was exchanging his cash - if the desks were even open! Well, we didn't think much of it as we expected one of us to be back for work in the next few years AND didn't realize how much he actually had in his pockets (about £50, not too terrible but more than we remembered!) We pulled it out for this trip and made the interesting discovery that nearly all of it was withdrawn, i.e. not accepted tender, so couldn't be spent!
Not something we're used to here in the States. When money changes it gets pulled out of circulation and we don't think much of it. In the UK you're supposed to exchange it for the new currency and after a certain point it's not valid anymore. Now bank notes are easy enough, the Bank of England guarantees all genuine notes indefinitely, you just have to exchange it at the
Bank of England Counter. We were also stuck with a bunch of withdrawn £1 coins, which are from the Royal Mint and has no such policy. For people with a UK bank account, it's easy enough to exchange, most banks will let you deposit them into your account. However, if you don't have an account, at this point you're fairly SOL since they were withdrawn 5 years ago. I got lucky and found a Barclay's bank with a sympathetic money exchange teller who was kind enough to exchange mine!
My long winded way to say, I wouldn't bring/withdrawal a bunch of cash. The exchange rate back to $$ isn't favorable, either at the ForEx counters or your bank on the return. And you never know what will happen to it later, so you don't want to hang on to it. Plus, some of the tourist places only accepted card payments and it was hard to find a line that took cash. I like having some as backup in case something goes wrong - like the one pub that disliked every single one of my husband's cards, even the true chip + PIN one

- but
just enough to get by.
What did you do for cell data? Did you add an international plan to your existing cell plan or get a local sim card for the trip?
For data I got a local SIM. Made it cheap and easy for the few local calls and texts I needed. The lowest price PAYG Sims at the airport were £30 but had unlimited calls, texts, and a large amount of data - like 100 GB. Way more than I needed for a week. I stopped at an
EE store near my hotel and picked up a £10 pack with 5GB of data, 500 call minutes, unlimited texts. (they had a promotion and gave an extra 15GB of data but I don't know how often they do that. I don't think I even used 2GB though) I couldn't text/call US numbers without adding credit to the pack but I just used an IM app to communicate with everyone over data. My husband was there for work so he just used his company phone, otherwise it would have been just £20 for the two of us. Cheaper than adding the international passes to our Vzw phones and only added an extra 20 minutes or so to my itinerary.