So I did some more digging and it seems I was in booked in U class, so that explains some of the difference (as thought I was a B fare).

I found this in the fare rules though:

UPGRADES - THE BELOW IS ONLY APPLICABLE IF
CHANGES ARE PERMITTED
-AT ANY TIME WITHIN TICKET VALIDITY THE FARE CAN
BE RE-ISSUED TO ANY HIGHER FARE IN THE SAME CABIN
RECALCULATED FROM THE POINT OF ORIGIN AT A CHARGE
AS APPLICABLE PER CHANGE PROVIDED THE CONDITIONS
OF THE HIGHER FARE HAVE BEEN MET AND THE
APPLICABLE DIFFERENCE IN FARE IS COLLECTED.
CHARGE APPLIES PER PASSENGER. PENALTY DOES NOT
APPLY TO INFANT WITHOUT A SEAT. IF THE RE-ISSUED
TICKET IS SUBSEQUENTLY CANCELLED THE ORIGINAL
PENALTY WILL APPLY.

Perhaps they've changed the fare for U class and recalculating based on round trip. Who knows... I'll check with my TA to see if they can do it, but the company doesn't like people to pay from their own pocket for changes so thats why I was hoping to do with via the call centre...

Thanks all!!

*edit*: So I just called the DXB call centre and they priced me GBP300 including the change fee... that is too funny. She was very quick to calculate it, perhaps only 20 seconds. When I called the UK CC the other day she took 5min to recalculate the fare. Very strange... so happy days, doing it this way around is cheaper than buying the miles even including the additional UK taxes owed!