eSims are super easy and you can get them ahead of the trip, set them up, but then turn them "on" when you land. All modern cell phones can handle them.
For "GPS", keep in mind it works ANYWHERE in the world as long as it can see the sky. Your phone will always know where you are as long as you're not in a tunnel or heavily obstructed place. BUT, it will only know the GPS coordinates for where you are, when in reality, a human needs those coordinated plotted on a MAP (overlay). So, for a phone, the options are 1) "pre-download the map overlays (in Google Maps)" which will give you a ready representation of those coordinates on a Google Map assuming the area was part of your downloaded maps or 2) have a data plan/wifi when using the Google Maps (or Apple Maps) and it will take the coordinates and constantly pull down the appropriate map overlay in real time.
Obviously, the upside of real time is that it enables all the smart stuff Google maps can do like routing for speed, avoid tolls, no highways, whatever. Plus it will always be updating traffic conditions and the names of stores nearby and links to their reviews, etc.. But with no or a dodgy internet connection, that turns to useless fairly fast. For me, I'm usually going to download a map(s) of a city for offline use, but I also generally have wifi via the esim or hotel. A while back, I didn't bother with an esim for a city trip, and relied on the hotel, museum, and cafe wifi - which worked great with the downloaded maps, BUT I realized it didn't work when out and about and asking Google Maps for the quickest walking path between two points since it needed the internet for that effort.
An "old school" Garmin/TomTom/etc GPS will come with regional maps already downloaded (same as cars used to). If you rent a car, it may most likely have all the GPS function you need built into it already or they may "rent" one to you for a fee. If you were to bring your classic GPS from home with you to Scotland, likely you would NOT have the UK maps on the device, so possibly would essentially have the same situation as Google Maps on your phone with no internet.