The thing with all of this is interpretation. The guidance
@nun_hunter quotes makes complete sense to me. However, and anecdotally, a number of the seizures we hear about don’t meet the threshold of “the threat of harm is so significant that immediate action is required, where justified and proportionate, … when any delay in taking action may result in serious injury or loss of life”.
It’s a strain to see how someone’s wife making a threat really creates a significant risk of immediate harm resulting in serious injury or loss of life. At most, it suggests a slightly higher risk profile.
As can be seen from the BASC link (and excepting other limited circumstances) the police only have a legal right to seize firearms where grounds would exist to revoke the relevant license. I’m personally doubtful that the situation the OP describes would cross that threshold (but could accept it might, with specific circumstances).
To the interested observer, it feels as though police use the request of voluntary surrender (and associated threat of a ‘black mark’ for being uncooperative) to undertake seizure where the threshold for revocation isn’t met.
I would have less of an issue with that if there was a formal process for this, including an expected timeframe for enquiry and a right of appeal. However, when forces are taking years to grant licenses (if they will at all) and where stories of long waits, with inadequate information abound, I fully understand why some shooters feel the situation is unfair, heavily biased against them and liable to leave them in indeterminate limbo.