Been there, seen it, done it. With a shot gun held on my SGC. So it was done by a telephone call to the licensing department.
Had a Webley .410 bolt action that had, as they all eventually do, stripped its front screw. It is a design fault. Again it would be an unsafe gun to attempt to sell on. So that was also explained to the licensing department.
Police arranged for the FEO to collect it from me (and give me a receipt) accepting it for destruction. Her words were "I'm actually quite interested to take this as it will give me practice booking in a surrendered firearm for destruction".
So it may be that your FEO is actually quite happy to have to do what may have been a paper exercise for real.
One last word.
DO NOT FORGET THIS....even though the police are taking it you will still legally be required to do a "Notification of Disposal" to them just as you would if you sold it on. Which is why you need a receipt and also why it is better done with a call to your licensing department to ask THEM how they wish you to proceed.