This is a modification of Zap's addon found here: https://www.curseforge.com/wow/addons/zapretroui
It is to replace the 8.0 default UI with the previous one. It also updates the xp text since blizz have broken it at the moment.
You can type /ui to toggle the options menu and adjust the position of the UI until it looks right. It's not perfect but it'll do!
The Allow Taint option will mean this addon will throw an "Interface action failed because of AddOn" error when you leave combat. This, however, stops the XP bar jumping around. Just ignore it, all is well =) Alternatively, you can disable this option which will avoid the warning but it will cause the xp bar to jump around briefly at the end of combat.
Thanks to Zap for laying the groundwork.
Alternatively, I have noticed a lot of people prefer the new UI but wish that pesky buttons bar was not there in the bottom right. In that case, check out the BagButtonHider3000!