No, I don't mean the single player content modding that people associate TES with. I mean UI modding that the majority of MMO's allow.

Being a fairly serious MMO player, I desire to be able to customize the UI any way that I see fit, for my specific playstyle/need. I want to get the most out of my experience and that requires being able to mod and resize the UI in any way I deem necessary.

I'll give some examples. World of Warcraft allows any form of UI mod that you can think up and create.

http://imageshack.com/a/img812/4961/s5yt.jpg----->http://imageshack.com/a/img266/1896/gnsj.jpg made myself via the use of over 20 mods. For the majority of MMO players, this feature is fairly important, as it allows personal customization for your own specific need.

Has it been confirmed whether or not TESO will allow UI modding?

Yes, they confirmed that UI modding would be permitted on the PC.

Source?

It is confirmed. If you insist on source on this one I suggest you google it yourself.

There's just too many interviews and stuff around the web to quickly find it.

You'll just have to take our word for it.