Is tea with milk is it as healthy as tea without milk?

Assuming you are not lactose intolerant it will provide some nutritional benefit. The milk will provide lactose, calcium, protein and fat. If the milk is vitamin D fortified, you will get that, also. How much depends on how much milk you add and what kind of milk you use. Check the nutrition panel on the milk carton and measure the amount of milk you add to know for sure.

It is unlikely that milk will cancel any of the benefits you might get from the tea.