Did you ask her why she said no, you can't unpack anything?
She may have been trying to be helpful in weighing your hand luggage, to see if you could shift a couple of kilos across, but have found that you were up at the limit and therefore the'no' was a practical answer.
£39 does seem a lot to pay though.
Read the link below, I would have expected you to pay 30€, assuming that your hand baggage stayed as hand baggage and was not checked in.
Lufthansa - Excess and special baggage
If you have the receipt, maybe a letter and request for a refund would be in order.