I just realised i made a typo.
Texturegroup should have been LODGroup :P
I can really see any other solution to this apart from playing around with the compression settings. AFAIK this is a good as they get as the lod size for UI is 4k.
As you suggested before, you save from doing this step a ton of time, you can change the .ini (after backing it up:P)
Change from line 107 onwards
If you cant edit the file just right click the file>Properties and uncheck Read-only
Sorry i dont have much more to help :/