Just returned this past saturday and flew with Britannia. Can give you both perspectives as we got a free upgrade to premium on the way out, and economy on the way back.
Premium: the seats were wider with big padded headrests, longer seat pitch (although if person leaned back in front of you, you still felt a little crushed!), free drinks (order 2 at a time though as they don't come around that often!), choice of 2 meals with wine and seatback tv screens with a choice of movies/tv/radio and a wired remote control. Headphones were free, given a small cup of orange prior to take off and a little pack that contained green socks(?), toothbrush/paste, hand cream and eye pads.
Economy: narrower seats, small seat pitch, pay for everything (headsets/drinks etc.), get the meal you are given (although must admit it wasn't too bad), only one film which you had to watch on overhead monitors
Obviously premium was nicer (especially when free!), however would I pay the difference? Probably not, would rather use the money while over in Orlando.
I would have no problems using Britannia in the future (and plan too this coming December). Staff were nice, planes very modern, no delays and a comfortable flight. Flying out and returning we landed ahead of schedule!
Dave.