For All You Tech Savy Phone Guys and Gals

Only if you:
1. Have roaming activated (some carriers offer decent roaming plans)
2. Buy a local SIM or an eSIM for the UK
3. Use public WiFi

A local SIM or eSIM is probably the best option, unless you're OK relying on WiFi for a short trip. Giffgaff is great for physical SIMs that allow calls as well as data; or check out one of the travel eSIMs (usually data only, but you cannuse something like WhatsApp to call friends and family).

Use public WiFi

Only do this if you have a VPN on your phone...I no longer use unprotected hotel wifi, and I have never used public wifi anywhere.

And I agree with the previous poster - get WhatsApp for texting and calling while abroad - but also only use it if you have a VPN. Any family in the US who you would want to stay in contact with needs WhatsApp, too.

What Simon writes is true, but not totally true...

GPS/location should work on your phone regardless of whether you have a carrier
or not. It is possible to download maps in advance for where you are going to be,
and then just turn on location once you are local. I suspect you can even download
the maps on your regular phone (I assume the A53 is a spare or something) and
then transfer them to the A53.

Offline maps

What you may miss is the ability to get real-time data on traffic, etc, as well
as checking on what the rating is for that restaurant you are walking by at the
moment. And if you don't download the most dense version of the map, you
may not get the detailed street-level info for tiny streets, etc. But it can work in
a pinch.

Just a word of advice, as I believe it's your first trip to London....
Totally non-tech....

Get a paper map of London now before you ever leave home, and study it.
Becoming familiar with the layout of the city will help you later as you are walking around in London using Google Maps or Citymapper, etc.

With those phone apps, you usually are looking at a small area, zoomed in on a couple of blocks or streets. You need to become familiar with the big picture beforehand. It helps very much once you are in London.

This is an excellent map, available from the Rick Steves online shop:
https://store.ricksteves.com/shop/p/streetwise-city-maps

Or you could pull up a Google map of London on your computer screen and study that.
Studying maps of the tube and bus lines before you go would be helpful, too.
Go to London with a pretty good idea of where things are and how to get around transportation-wise.
It will help you tremendously.

Great information. My carrier doesn't have an international plan. But like I said, I not going to be calling nor texting. I want to take in as much of our vacation time as possible. Just want to use phone as a gps to navigate as needed. Bus schedules, tube schedules etc. I checked and my phone isn't compatible with esim also. I'll need to get a UK sim card at Heathrow when we land?

We are staying close to the Tottenham Court Road Station. I did print out a map of the bus and tube lines. Looks like we are in a pretty good spot. getting around should be relatively easy. I just want to try and avoid any time consuming mistakes while moving about the city.

Look into giffgaff. They will mail you a free sim card with activation instructions. Download their app, set up an account and buy a “goody bag” of data. You will get a uk phone number but you don’t have to use it.

I'll look into giffgaff. Three years ago, we went to France. I bought an Orange Holiday Sim Card that's supposed to work in Europe. It didn't work. That was 50 dollars down the drain lol. Of course, I had an older (Samsung Galaxy Sky) phone back then. Maybe with this newer 5G phone, things will be different.

Another vote for Giff gaff which I have used twice before.
Very efficient.
Activate it before you leave home ,then take out the gg sim.
Put the gg sim back in just before you land in the UK, and it works right away.

Should only cost £10 or £12 on giffgaff for lots of data plus UK phone calls and I think its unlimited UK txt.
I have used them them near a decade

I have never used VPN and see no reason to start, assuming I restrict my sensitive usage to times when I'm not on public wifi.

I'll try a GG goody bag for a month. Says up to 5GB roaming in the UK. Will that be enough for a few days in London? In London, I will use the phone for navigating and checking bus and tube schedules as needed. In Ireland, our time is already planned. Titanic stuff and a couple of tours. I probably wont use my phone at all in Ireland.

Unless you spend hour upon hour streaming video ,5 GB should be plenty.
In my experience of activating a good number of thier Sims ,its near enough instantaneous.
It used to be the case ,£10 was the minimum to activate that was typical of the main four mobile providers

I found myself using more like 15 GB monthly. No streaming video, but I did run Qobuz music a fair amount.