If it's the original radio a Nissan main dealer should be able to obtain the code from the manufacturer's database, using the registration number or VIN. (i.e. there should be no need to remove the radio). The typical charge for the service is around £15, probably plus £3 VAT. (That's what Ford dealers usually charge. I can't see why Nissan should be any dearer).
Try a different Nissan dealership:
http://www.nissan.co.uk/#tools/find-it
Alternatively, if you can work out how to remove your radio, there are online services which can provide the code from the radio's serial number. This one has plenty of good reviews. (They can also sell you a special tool to remove the radio, should you need one, and their website tells you how to input the code):
http://www.radio-code.co.uk/
Chris