Strange, My daughter requested the MAC from Orange in order to change to BT, no problems it was given to her and emailed.
Ring again, stamp your feet and insist, also suggest reporting the situation to the nominet register. You will get the code, just tell them you are changing providers.
You could also speak to your intended ISP.
As to the kit, well if you agreed to the contract or change of contract you may be stuck with it. But as they have agreed to refund you and you have offered to send the equipment back this should be as simple as the colour orange. If they mess about with the kit, consider it a gift.
As a mater of interst I called Orange (my current ISP) and asked for my MAC code, got it with no problems.