Mobile service

Without being able to unlock your phone, you can buy a used phone on ebay or a trackphone, and get a sim card by GiffGaff or similar, and pay $10/mo for just the time you're in Scotland. If you have a retired phone, check if it is 4G, and use it. My carrier (US Cellular) is $10/day if I turn it on while in Europe, which includes too little data to use Maps or data intensive apps. The downside of the second phone solution is that you have a new phone number, which might be a headache if you're a heavy phone user. I only use the phone to find places when on foot and call for dinner reservations which is easier with a UK number (that comes with the sim card). There's lots of discussions on this topic here on the forums.

Agree with all the above.
Every UK town centre tends to have at least s couple of mobile phone shops selling second hand phones too
The other shop to look at is Argos,new basic smart phones from £40 to a very decent one like the Honor at £120.
With the outrageous tarrifs US carriers extort ,it makes good sense.
I used to to look at my cousins " deal" with AT n T and laugh saying for £10 you can get multiple the allowances

Phone plans for tourists are cheap in the UK. Get a second phone if yours is not going to work on their SIMs. It won't matter where you get it from, in general. Also, I would absolutely do my SIM shopping in person. They will usually give you a great deal from their dedicated shop. ie. O2, or Vodafone, et al.

My phone is not unlocked.

Have you tried asking your carrier to unlock the phone? In many cases, they will upon request. For example, both Verizon and T-Mobile will unlock a phone after a short period of time after purchase (usually 60 or so days). That would give you the ability to go with an eSIM data plan like Airalo.

It turns out that my Verizon plan earns one travel pass day per month. They expire after one year, so that means I will have 12 travel pass days for free at the time of my trip! That is perfect!