Nedbank Forex pounds sterling is not accepted in the UK

We went on holiday to the UK and exchanged our rands for pounds sterling. Nedbank gave us discontinued currency. We couldn't use the 10 pound notes whilst travelling. We wrote to them to complain and they failed to accept responsibility, claiming that their notes are fine. No shop would accept the notes, we had to withdraw money while we were there and incur added fees. The notes have been discontinued for about a year now.