Cordylines can be quite hardy in some regions of the UK ie the south and west coasts of England and areas that get the bennitit of the gulf stream like many parts of the west coast of Scotland and also much of Ireland but in recent years it can be seen to grow in a much wider range.
For your one to grow that fast, I think it must be well sheltered from cold winds etc.
The flowers are very small and insigifisent but grow in large clusters, so I guess it may have flowered and set seed without you realising ?
After flowering the plant often becomes branched but still retains it's exotic apperance. the dead flower stems can be cut off once finnished to keep it looking tidy.
Good Luck Tbird+