What did you forget in planning for your UK trip?

Just to make sure - have you got ETA? And made sure your passports are valid for six more months? Have a source of payment and a backup? I think almost everything else can be worked around.

Have a wonderful trip!

Edit: Johnew points out that for the UK, you do not need passport validity beyond your trip duration. Thank you John!

I posted my England/Ireland trip report a couple days ago. It has a link to my packing report. Maybe they both will help you. I can’t think of anything I forgot that I couldn’t have bought there.
https://community.ricksteves.com/travel-forum/trip-reports/london-south-of-england-ireland-trip-report

Do you have money belts? Are the apps you need for transportation, maps downloaded on your phone? Did you set up an International calling plan? Look at the Bloomberg Connects app to use as a tour guide.

Passport. ETA. Credit card. The rest is noise.

Hi MapLady, yes we got our ETAs a couple months ago and passports are valid. My wife and I each have a couple credit cards and we will withdraw some cash when we get there. Good point, everything else can be worked around!

Hey Horsewoofie, I saw your trip report the other day it was very helpful! What apps did you use for transportation there? I have Google maps and CityMapper for navigating, I also have Uber and Free Now (which I read about in the Rick Steves guide book).

I wouldn’t be so quick to take cash out. I made two trips to the UK last year and did not spend a but of cash. UK is very cashless in my experience.

A swimming suit...friends had a timeshare that had a spa and a pool. Some of the castle hotels have a spa & pool, also.

BethFL, good to know. That was our experience when we went to Iceland. Even here in the U.S. I rarely ever use cash so it makes sense.

Kathleen, thank you! We actually had planned on packing swim suits but forgot.

I was in Scotland earlier this month. I wish I had packed more light clothing. It ended up being warmer than forecasted, and I only brought jeans and a couple short sleeved shirts. My multiple sweaters were not needed. I wish I'd had one pair of shorts.

As for cash, you don't need it. We had leftover pounds from a previous trip to London. I only used it once because we wanted to get some coins for a nearby bathroom. After that, all the other toilets we encountered took card or were free.

Have a great trip!

Once, many years ago, I made the mistake of taking continental European plug adapters instead of the UK ones. I was able to buy one while in London.

You do not need 6 months more on your passports. You just need enough validity to cover your trip. It’s a moot point anyway if you’re leaving tomorrow.

Jay, sorry to be slow to answer your app question. I already deleted the apps I used. But I'm pretty sure these were the apps I downloaded: (Plus similar apps for Ireland including Air Lingus)
National Express for bus, Bath to Heathrow
National Rail for train planning, routes you to the train line for ticketing
Southeastern Train for train, St Pancras to Canterbury
I downloaded tfw/TfW (Transport for Wales) but didn't use the app or their trains.

If you have an iPhone, do you have Apple Wallet set up with one of your credit cards? If you will be in London, it's a convenient way to tap in and out of the tube and in only for the bus instead of buying an Oyster Card. You can also use your tap to pay credit card. But be consistent which you use for the best rates.

Have a great trip!

You probably will not need any cash, I was there a couple weeks ago and never needed any cash. Make sure each person has their own contactless method to use the tube if you're going to London (for kids, may need to buy an Oyster card at the airport train station). It was also quite hot when we arrived, so I was glad I had summer weight clothing. I also had a need for an umbrella due to a torrential thunderstorm. I had the umbrella, but it was in my suitcase at the hotel. In London, CityMapper is excellent with directions - if you follow them! Also if it's hot it will suggest routes with air conditioning.

Have a great time!

@Johnew, good to know. Thank you. I edited my comment.

And just as an FYI - in an urgent situation, if you go in person to one of the US Dept. of State offices, you can get a passport renewed within a day. My relative who was denied boarding because of insufficient passport validity managed to get theirs renewed within a day in San Francisco and could fly the next day.