Sim card

That kind of depends on your phone as well as your regular carrier. The easiest, but most expensive typically, is to pay your regular carrier for service abroad. That way you also have your home phone number. Verizon, for example, charges $10 per day or $100 per month.
You could also buy a physical SIM card in the UK. This effectively makes your phone a UK phone so you don’t get calls on your home number. (You can set WhatsApp and Signal to maintain your regular number). And it is a lot cheaper. You do need to be sure not to lose that home SIM card!!!
However, some of the newer phones, like, I believe, the iPhone 15, do not have a physical SIM card but rather an eSIM. So to travel you need to purchase an eSIM. I’m in the process of learning about that so I’m not one to give advice there.

Typicaly a UK Sim with lots of data will be £10 to £15 at absolute most.
The packages Verison and co offer you guys are eye wateringly expensive .

At least on Giffgaff you could add a little extra credit above the best value goodybag and it was 3p to call the US landline or mobile and 8p per txt
But with a " goodybag" which includes hundreds of mins of UK calls plus unlimited UK txt ,whatsapp,telegram ,Facebook Messenger would be free of course in the Goodybag.

You can pick Sims up very widely and in my experience activation takes minutes

One way to conveniently deal with this is if you have an old phone in a drawer at home bring that and put a U.K. sim in there. Then you can use that for any local use here but you still have your regular phone for anyone to contact you and you can use WiFi if you need it to call home. This has worked very well for us in New Zealand three times.

If I put a SIM card into a spare phone at home, does it have to be an unlocked phone?

Yes, it does have to be unlocked to use a SIM from another carrier.

Unless you have a very cheap or old phone, it likely has the ability to use multiple SIMs. And to use eSIMs, which don't require opening up the physical phone. This Reddit thread contains some useful information:

https://www.reddit.com/r/eSIMs/comments/14dz7pf/can_i_store_multiple_e_sims_on_one_phone/

If the issue is Google Maps, you don't need a sim or even data. Download maps covering where you are going directly onto your phone. Then the GPS plots your position right on these downloaded maps without requiring any data. For details, just google "downloading google maps".

If the issue is texts, any sim or data plan seems a waste compared to just waiting until you are on wifi to send your message.

The only need I have ever had for data while in the UK is to use Citymapper.

If one downloads Google maps over wifi and doesn't have a data plan, one won't get any ability to get driving directions. That alone makes getting a data SIM worthwhile; they're dirt cheap.

If the cost of a data SIM bothers you, you're not able to afford international travel.

The cost of a SIM card does not bother me at all, it is the technology that gets me into trouble.
I am well up into my 80’s and these things unfortunately are not as easy as they used to be

I understand entirely your comment about things seeming harder than they used to be. Sounds like me!

I'd honestly suggest you visit your current cell phone provider and tell them what you want to do. Let them know that your phone needs to be unlocked to allow you to activate an eSIM from another country. The tech staff there should be able and willing to help you set things up; if not you can wait until you get overseas, buy the SIM there and ask for their help.

Thank you for your help. Much appreciated

Here are a few things I was surprised at when last fall I used a purchased a 30 day UK sim card at Heathrow for my old 2016 iphone SE.

As far as using it for google maps and email, it worked fine. Where I ran into problems and confusion was texting back home to friends in the US and Canada, as well as trying to phone them, and finally not being able to create a secure hot spot that I could use for online banking.

After several failed attempts to text friends back home, I emailed them and told them to text me both at the new sim # as well as my normal number. Both were hit and miss. Also, when I wanted to call home, the UK card would not let me, so I had to put the old card back in during the time I wanted to phone home, which meant a 16 dollar charge. I also wanted to do some banking to transfer money to my credit card using my travel laptop but the UK sim card would not allow me to create a secure hotspot. (using public wifi or even hotel wifi for banking is not a good idea) Again, back in goes the regular sim card and another 16 dollar fee.

It's quite probable there are some easy workarounds for these, but I couldn't find them.

The simplest (but most expensive) way to do it is to get international roaming put on to your plan by your existing provider. If you aren’t confident dealing with technology and cost is not a major factor for you then I would just take this option.

Since Brexit we have to pay roaming charges when we travel to Europe and I have tried various esims and cheap options and while they do work eventually there has been quite a bit of faffing about involved.