Your point about the specific upgrade fee from Jersey is a good one; however, BA just seem to level the flat fee and then deduct whatever charges they need to pay. On a recent trip to venice it was shown as £45.00 U/G £5 tax... (when the fee was £50.00!!)
Most outstations charge 75 Euros but it isn't always advertised, even when available....
I wonder how many people U/G on the JER flight??? I know that at peak times the CE cabin has been back to row 9 at least....