IPhone in U.K. To sim or not

You do not need data to use a maps app.
As long as you download the maps (data) before you leave home, or on WiFi (in the hotel), you can use the app and view the maps without any data connection.

I use maps.me ( https://maps.me/ ) which totally works with no phone service.

Maps? We both have iPhones and a data plan, but we use paper maps in London. It is really nice to have the big picture and plan one’s own route.

Agree that paper maps are useful (of course).

Disagree that having a phone without cell service is a good option for navigating. It may work sometimes (when you can poach a wifi connection, or when you have previously downloaded some limited map data and that's all you need), but that may fail you when you want or need it most.

I believe most (though not all) new iPhones are unlocked these days. Check if yours is (ask Apple if you have the option to pop in a SIM while abroad; or since yours is new, ask the people who sold it to you).

Getting a SIM for cellular service in the UK should be a simple, trivial, inexpensive option. I'd go that route. I certainly wouldn't buy an "inexpensive phone" while there just for this...any phone will still need cellular service to make full use of navigation apps so why not enable the phone you already have?

A pay-as-you-go SIM card pre-loaded with a data deal is not expensive... plenty of data for a week for about £10. Look to the major carriers EE, Vodafone, and Three, for good coverage and 4G LTE. There will be a bunch of SIMs on offer from a variety of sources, but I’d stick with the main carriers. Less confusing.

Personally I’ve used EE several times, they have stores in most towns in the UK, since you didn’t mention where you’ll be. https://shop.ee.co.uk/price-plans/free-sim

Carphone Warehouse chain sells a selection of SIMs from these companies, too.

My phone is new, will unlocking it effect my warranty.

Unlocking a phone has nothing to do with the phone itself as it is a change made outside of the manufacture of the phone. You need to check with who sold you the phone and service to see what it the charges will be to unlock it.

Who is your carrier? And how much will they charge for data overseas? I'm sure it's pretty expensive.
iPhone X's have dual sim capability, does your iPhone use the E-sim or the actual SIM slot? or is too much technical stuff to deal with?

We used Giff Gaff https://www.giffgaff.com
when we were in the UK. Once you get there you can pick up a Giff Gaff SIM chip from a phone provider there, and even at the airport. They can install it for you, if you don't know how or don't have a paper clip.
The cheapest GIFF Gaff plan is £6 for a month for 500MB data, 300 minutes and 500 texts plus you need to add a £10 goody bag for calls and text not covered. (Texts and calls within the EU are included. Texts and calls to the US are not- hence the need for a goody bag.) So you are paying £16 for a month.
How much is an inexpensive phone? Do the math. Which works better for you?

Most iPhones are unlocked at time of purchase from apple. Typically iPhones purchased from ATT are locked.
Unlocking your iPhone does not affect the warranty.

If you hung on to your old iphone, take that with you and put a sim card in it. That’s what I did in London last year. I went to a Carphone Warehouse by Victoria Station and the sales guy even installed it for me. I brought my iphone 8 as well but put that on airplane mode, turned off the data and used wifi when available. This way my friends and family can contact me using my regular phone number. The new phones that have a dual sim sounds very intriguing! If you use that, let us know how well it worked for you!

DeVon,

Comcast (Xfinity) appears to offer some international plans but not the same coverage as you'd have at home - https://www.xfinity.com/hub/mobile/use-phone-international-texting-calling . If you're going to be in the U.K. for a week, you might consider.....

• Call Comcast to verify that your phone is unlocked.
• When you get settled in your hotel after arrival, go to the nearest Carphone Warehouse and one of the rep's there will be able to set you up on a plan that matches your needs. DON'T forget to store your Comcast SIM card in a safe place, so that it will be easily available when you return home. You might want to pack along the small tool that pops the SIM tray open.

Keep in mind that you'll have a U.K. number while the other SIM is in place, and you'll be subject to the pricing and terms of the UK plan.