Another thing that's probably familiar to a New Yorker is the cost of hotel breakfasts. Unless you book a room that comes with breakfast, the cost is ridiculous. But like NYC, there's always a little breakfast place nearby that'll run you 1/3 of the cost and be just as good. Scope one out when you get there.
If your flight is overnight, be prepared to not have access to your room until 3 or 4PM. That can suck if you don't sleep on your flight and still need to function through the next day until your room is available.
All the museums are free there. So if the weather isn't great, take advantage of that. And if you want to browse the big stores, check out Selfridge's. I like it way better than Harrod's, although Harrod's is also cool to walk through.
You can buy SIM cards for your phones that will allow them to work over there. We usually find a plan that has unlimited texting and calling and about 15GB of data for 15 BP per phone. I like EE (
https://ee.co.uk/), but there are plenty of other telcos there. If you do that, have the guy setting up your phone program your new numbers in so you can add them as your contacts while you're there. And make sure the tech labels your stateside SIM cards properly so you know which is which when you switch back.
Before you leave the US, download the Customs and Immigration mobile app. It'll save you a ton of time when you come back into the US. You only need it on one phone. Make sure you switch back to your stateside SIM card before using the app.
https://www.cbp.gov/about/mobile-apps-directory