The credomobile.com page has great detail regarding your options. Click on support and look for info regarding international usage. Your best bet for the least expensive cellular phone and text and cellular data would be to purchase a local alternate SIM card in the UK to use in the phone in place of the Credo SIM while you are in the UK. If you only plan to use little or no talk and text while there you can simply have the phone enabled. It can cost over $1 per minute to call that way, though. For my last trip to Europe, this worked fine, as we were nearly always in reach of free wifi which allows for communication using apps like WhatsApp rather than using regular talk and text.
I decided I needed a break from my phone so I used my iPad to communicate. Wi-Fi was everywhere.
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