Dats access with SIM card?

A local SIM card will give your phone access to cellular data which allows you to check maps, email, browse the web, etc. You can do the same when on WiFi. The advantage of the SIM card is that it allows you to do these things when out and about without having to be on WiFi.

I always wait until I am in town to buy a card and plan. When I checked the shops at Heathrow the choices were limited and generally more expensive. In London I have used EE and Vodafone. Both services were good. But I think their pay as you go (prepaid) have extra charges for roaming in the EU.

Giffgaff comes up in the forums. I believe they will ship a SIM card to the US and their plans have EU roaming.

My plan is to use lycamobile UK for my next UK trip. My new phone has no physical SIM slot and Lycamobile offers E SIM.

A SIM card gives you data on your phone when you are within reach of a cellular tower. Don't confuse that with internet connections using wifi.

It may help to realize that you have a SIM card in your phone right now. It is what identifies your carrier, mobile ID, and subscriber info.

Getting a new SIM just makes you a temporary subscriber to a new carrier.

As noted, wi-fi will give you the same capabilities here or there.

Giffgaff now offer eSim too.
In the UK £10 will get you generally unlimited UK text ,many hundreds of mins of UK calls and 20 to 40 GB of data.
You can opt for just a one off 30 day service
I use EE and Giffgaff
Set up takes 5 mins

Momfor4, I agree that a local sim is normally much cheaper than the daily roaming price from Verizon or even AT&T, but it can be a bit of a headache to get activated. For my backup phone, I've recently started buying sims from Ebay sellers and activated them before I left the US.

Important thing to note: If you get a sim or esim from a European carrier or MVNO, you will then have a new phone number from that country. If you have a dual-sim phone (or sim/esim), you can leave your AT&T sim in, and add another sim. Then I think you can make phone calls using your original AT&T number, and doing "Wifi Calling" with data from the foreign sim (someone correct me if I have that wrong).

If you wind up calling people back home with the new phone number, they might be charged a per call surcharge if they call or text you back. And if you text them, they will not know it's you unless you state your name right away.

I used to have AT&T before I switched to TMobile, and what my wife and I did was to buy a 7 day international pass for $35 on AT&T.

This episode of RS Monday Night Travel Talks covers the basics of cell phones, SIM cards, wifi, and more. I thought it did a good job explaining the basics and perhaps it will clear up some of the confusion: https://youtu.be/hqHMHmA7I6I?feature=shared