Can anyone suggest a travel SIM card that works well in the UK? Is it better to wait until I get there or buy in the US before I go? Thanks.
Look at giffgaff. They will send the sim card to you in the US and you can install it (via their app) then activate on arrival. Their rates are really good for data/unlimited talk and text within the UK. We had no problems with network coverage. You will pay to make calls back to the US, but we used google voice and facetime audio for that. Search the forum for other user experiences with giffgaff.
https://www.giffgaff.com/
Another vote for giffgaff.
I ordered one in advance, it arrived within 10 days, and I set it up at home.
It worked immediately upon landing.
I was reading an article recently where they absolutely positively wanted you to swear you would NOT buy a SIM card at the airport. Ironically there was a vending machine at baggage claim for just that purpose.
There are plenty of phone stores in London, you could pop in and pop out with a SIM card with no problem.
You can walk into more or less any corner store or supermarket and pick Sims up.
Activation is easy and takes at worse 5 mins from experience..
I have been dual sim for 10 years and a giffgaff user from them starting out.
Buying in a shop in the UK can get you help in setting up service if needed. It will likely get you a better selection of service providers and plans. I’ve looked at the vending machines at Heathrow and they have limited choices and are more expensive than those offered in the shop
I don’t need cell service the moment I land. So I pop into a shop in London. I’ve had good success with both vodafone and EE.