I did wonder about this before I travelled in PE,but it is as Rob says.People are usually in casual/smartish (just like you normally would when you travel) but also to people in scruffy shorts +T-Shirts
Far be it for me to say how to dress on a plane as I will have my normal trainers, tee shirt as normal.
However we have booked premium economy for later in the year so I decided to pre-book our seats and according to the Virgin Atlantic representative I spoke to, my family is on the Virgin system as being booked onto our flight as Club Orlando from Manchester, which is basically Premium Economy only using the 1st class seats,as Virgin don't run 1st class from Manchester. This costs an EXTRA £160 each way per person