Having tried both I would disagree. Shelf stable UHT milk doesn't taste sweet to me at all. That is likely because lactose free milk is made by adding a lactase

Having tried both I would disagree. Shelf stable UHT milk doesn't taste sweet to me at all. That is likely because lactose free milk is made by adding a lactase and then using UHT pasteurization to stop the chemical reaction (and extend the shelf life). This means the lactose is already pre-converted into glucose (and galactose) in this milk. Shelf-stable UHT milk doesn't have lactase added and isn't sweeter than normal milk. At least in my experience.