When you fill and print a voucher for Amazon do you have to put the pound sign when writing the amount?

Assuming you're talking about a 'print at home' gift certificate. The form you fill in on-line already states the currency is in GBP (or Great British Pounds). Therefore - you just insert the value into the box - and the certificate will show the currency.

If you click on the related link - you'll see the Amazon gift voucher page. Click on the 'example' button - in the new window that opens, you can see that it shows the pound sign inside along with the value of the certificate.