Cell phone/GPS

TMobile has a great international plan--you may wish to do a temporary upgrade though (that's what we have done in other countries--I navigate with google maps, husband drives). I don't know how well Scotland's rural coverage is, so that is the real question. If you don't want to fuss with the phones, getting car with GPS is an easy alternative.

I’ve used the T-Mobile data in Sicily, Wales, Germany, France, Portugal, and Spain and have had no problem navigating with Google maps. I do have maps downloaded for offline use for those instances where there is no cell service. On my last couple trips to the UK the rental car did have a navigation system, but there’s guarantee they all will.

I have no idea what TMobile's international plan is, but my carrier charges $100/mo or $10/day. I buy a SIM card (or eSIM) online from Giffgaff for $10 for the entire month (15gb, slightly more for jumbo data). Having a new phone number is fine with me - the last thing I want to do on vacation is talk to Medicare salesmen or Microsoft Tech Support. Of course you need an unlocked phone. Maybe your old phone?
And having a UK number makes dinner reservations etc. easier, since you're more likely to get return calls.

The T-Mobile Magenta 55+ plan includes unlimited data in the UK and many other countries. I've used it for GPS in Ireland and France, and I will this summer in Scotland. I navigated in France with Google Maps, plugging in origin and destination to get spoken road directions. It was surprisingly easy and without it I'd still be wandering through the roundabouts. ;-)

You need to know which specific T-Mobile plan you have. And you may find yourself in some remote places where there's no cell service -- but my impression is that GPS will work anyway on Google Maps.

FYI, all of the cars rented via Celtic Legend include SatNav. We had a fabulous experience with our Arnold Clark rental car obtained via Celtic Legend. For on the ground navigation, I got an inexpensive data plan with EE. Sorry, but know nothing about TMobile. All the best!

Just trying to determine if we can drive around on our own.

It is possible to drive without a phone or gps. We used to do all the time in the old days. You could get a map.

I don't know how well Scotland's rural coverage is, so that is the real question.

Scotland's cell phone coverage, including rural areas, is probably the best of any country I have been to recently. It evidently spent a lot of money upgrading the coverage a few years ago and it really shows. I never ever lost coverage in the 3 weeks I was there, and I was using GPS (via Google maps) for about 2 of those weeks. I even had coverage when I was out on a boat touring some islands.

FWIW, I had T-Mobile and was very happy with it. Keep in mind that T-Mobile gives you 5G of data on the international plan, so once you use that up, you drop down to 256MB. I had it available, though, until the last few days I was there. So a lot depends on how long your trip is.

One thing T-Mobile offers, if you do run out of data at an inconvenient time, is a 30 day international mobile pass that will give you an additional 15G of data for 30 days. I recently did this in England because I started my trip at the end of a billing cycle, so had less data to use. I paid $50 for 30 days and it was well worth it. But I was in England for 5 1/2 weeks. I did not need that when I was in Scotland for only 3 weeks.

And you may find yourself in some remote places where there's no cell
service -- but my impression is that GPS will work anyway on Google
Maps.

GPS does work on Google maps (or any Sat Nav app) without internet. However, you must have internet to start it up. Once the GPS has started, you're good to go even if you lose the cell service. I had a few problems with that in England where there was zero cell phone coverage, and I would be stuck in a place without being able to latch onto my GPS. So I did have a map with me just in case.

But you should not need it in Scotland—as I noted above, their cell service is great, and I always had service, even in the Highlands and Hebrides.