I have heard before that UK guests can't change a GTY cabin once assigned.
What surprises me more is that US based guests are able to change it. What's the point in getting assigned a GTY cabin if they just let you change it afterwards?! I feel the policy should be consistently applied of not being able to change it worldwide but it's not.
Sadly, picking a GTY gets you a lower price, but no flexibility if you aren't happy with your cabin assignment.