Hi shakiraMoore,
Thank you for reaching out Microsoft Community and for checking with us about the Surface Pro X.
Surface Pro X has LTE advance functionality wherein you can use simcards for celllular data and mobility purposes. It doesn't really show a detailed specification as to what simcard should be used so I believe it can be used with different simcard in UK. See this link: https://support.microsoft.com/en-us/help/4056486/surface-insert-sim-card-surface for details.
On the other hand, you basically need to check if that certain application can be downloaded on a Surface Pro X because it has a different processor than those Surface Pro devices. It has an ARM based processor and this means that the application to be downloaded should be compiled to x64ARM or should be the x86 bit version of the app. At this time, Surface Pro X will not install 64-bit applications that have not been ported to ARM64, some games and CAD software, and some third-party drivers or anti-virus software. So, there's a need for you to check with the application developers to see if the app has a newer version that runs on ARM processor because it would be a waste purchasing the unit but the application you like won't be install on this device. See the Surface Pro X Tech Specs for details and I also would like you to check the Surface Pen you had in Microsoft Store online, there you can check the tech specs if this is still going to work with Surface Pro X. Sharing you this **link**as an example wherein you can check on tech specs the compatible device for this pen version.
If this answers your question, feel free to mark this post as an answer. Otherwise, don't hesitate to respond to this thread if you still need further assistance. I am always here to help.
Kind Regards,
Crest_Z