I use skimmed milk so that is 350ml per day. I only use it in tea or if I am having oats for breakfast on some days.

If I don't use all my milk allowance can I then use natural yogurt to make up for not using all the milk. I like having the natural yogurt (2 tbsps) over a cut up orange with ginger and cinnamon sprinkled over. A very refreshing dessert or a breakfast.

Your milk is your A choice, which is a recommended amount to use each day. Natural yogurt is a free food, which is classed as an unlimited food, so you can eat as much as you want each day.
If you don't use all your milk, it isn't an issue. You don't have to use all of it, it's a recommended limit.
And if you want to have yogurt, you can have that as well as having your milk, you don't need to choose between them

Does that make sense?

Thank you Sarah. That is good news and it has certainly cleared that up for me.

I use the natural yogurt from eating with fruit, as a base on toast or ryvita so that the tuna sticks to the bread or ryvita, a tsp on a jacket potatoe with seasoning sprinkled over or over salad or even mixed in with pasta to make it a bit creamy.