There are two National Express bus stops either side of the road outside of the Union. The one nearest the Union is where you will want to get on to go out of the city. When you arrive back in Portsmouth, the bus will drop you off on the opposite side of the road by the University Sports Centre.
There are 3 routes that stop at the University bus stop:
You can buy a National Express ticket directly through the Union Shop at a student rate, or alternatively you can go online with an NUS Extra card to get an even greater discount.