We did this several years ago and, although things may have changed a bit since then, we've booked to upgrade again for our trip this July, so we can't have thought it was too bad, can we?
I have to say the 'enhancements' aren't that brilliant but, with DH at 6'3", it makes the flight more comfortable for him.
He has been so squashed on other flights (including economy with Virgin and BA, not just the charters) that it's worth it for us all to upgrade even though DDs and I are all around 5'4" and not really in need of a lot of legroom! However, we wouldn't be prepared to pay x 4 passengers for the schedule airline premium upgrades.
When we last travelled Britannia Premium, I remember the seats being a bit bigger than economy (but not much) with some extra legroom (again, not much). The configuration in that part of the cabin was 2-3-2 (or maybe 2-4-2) instead of 3-4-3 in the rest of the plane, I think. The drinks throughout the flight were free but the upgraded meal service was a complete waste for us as DD1 was suffering from her usual travel sickness, so didn't eat, and DD1 and I are vegetarian and there is no upgraded vegetarian option! In fact, I seem to remember they messed it up completely and we weren't even down for veggie meals at all!
(* see edit below) DH enjoyed his though

The seat back TVs were nicer than having to look over the seats in front to see the one in the aisle.
To be honest, I don't get on plane to enjoy a slap-up meal or first rate entertainment

As long as we arrive safely and manage to have some degree of comfort for a few hours, we're fairly happy. I know others like to feel the flight is as special as the rest of the trip, but we see it as a necessary means to getting to our destination.
(* Just looked back at my trip report from then and remembered there was also no child's meal available in Premium! I'm hoping they've got their catering arrangements sorted out by now!)