I flew with Britannia in December in normal cattle class. To get to our seats we walked through the premium section and I have to say I would have been really cheesed off if I had paid the extra to sit there. There really didn't appear to be very much extra leg room at all.
I have also flown Virgin PE and it was well worth the extra.
If you have First Choice flying from your airport I would recommend their new premium cabin which is starting at the start of may. The seat pitch is 36" (Virgin PE is 38" to give you some idea) and you get seat back tv with 40 channels, free drinks, priority boarding and loads of other things.
If you have to go Britannia, you'd be better going for emergency exit seats in normal economy than the premium seats IMHO.