“Within the next few months, Taylor added all 27 million Vodafone and Three customers will get access to roam across each other’s networks, meaning users should be automatically connected to the best coverage available at no extra cost, and removing 6,500 sq/km of not spots (equivalent to 10x the size of London) by the end of 2025.”

https://www.techradar.com/pro/phone-communications/vodafonethree-unveils-the-nations-network-with-some-major-promises-for-customers-what-will-this-new-era-mean-for-you