So there's a nice guide on how to update the IDP sigs for an SRX which does not have internet access to download them.
Problem, what do you do about updating the IDP license. So far, the only method I can see is for the device to have internet access. So why not have a method of obtaining the license offline, especially if you recognise and write a guide on updating the IDP package offline.
TAC did offer to provide a license key but I found the answer I really wanted.
Although not mentioned anywhere in the documentation, if you go to Juniper / Support / Tools / Manage license keys / Find license keys / Generate license keys for subscription entitlements, you can enter the serial to generate the license key on text file for offline installation.