Hello, welcome to social slimmers and SW!
You are correct with your porridge being your B choice, but your milk is your A allowance, regardless of how you have it (milk in porridge, a glass of milk, cheese on a potato, etc.)
I've known of a lot of people who make their porridge with water rather that milk to save their A allowance for lunch or something. Personally, I don't like it with water, I have to use milk!
Be aware that there are only certain Oats So Simple porridges on your B list which are the multigrain ones, or the Big Bowl Original 39g sachet. The others don't contain enough fibre to count as your B choice.
Don't worry, SW takes some getting used to initially, so feel free to ask anything you want, even if you think it sounds silly... I pestered my first consultant something rotten trying to figure it all out