I’m looking to get the GL-X750 but I’m not sure which version to get which will work with ATT LTE as well as pay as you go UK providers like O2. Is that possible?
I can’t even get one, out of stock 
It doesn’t seem able to work both of in US and in UK.
If you want to use it in US, EP06-A is recommended, while in UK, EP06-E is recommended.
You can get an x750 with the A module for example and swap out the module for the E version when you travel.
You can buy the spitz now from gl-inet store with the EP06-E modem inside, and then purchase this Quectel EP06 miniPCIe - optimized LTE Cat 6 Module ver EP06-A for America for when u want ATT