Im new to Unreal myself, but my first attempt would be to create a Game-mode Boolean Variable Array. You have 8 levels so you would have indexes 0-7 and you could set the Boolean for each index indicating if level is unlocked. The array would be set on begin-play reading values from a save (Never done this yet). Your UMG level icons would be variables and you could set the image according to the Boolean value in your array. You could get then enable or disable interaction according to your array Boolean?