Egypt was once a Christian nation. Today it is roughly 90–95 percent Muslim.
Syria was once a Christian nation. Today it is 87 percent Muslim.
Turkey was once a Christian nation. Today it is about 99 percent Muslim.
Sudan sustained Christian kingdoms for centuries. Today it is about 97 percent Muslim.
Iraq, was once home to large Christian communities. Today it is about 97 percent Muslim.
These shifts did not happen overnight.
They followed conquest, sustained pressure, conversion, and the gradual erosion of Christian majorities over generations.
If you assume nothing comparable can occur in the United Kingdom, the historical record suggests you are mistaken.