I have heard this from several individuals first hand and seen the emails from a Police Inspector who made the decision to decline it based on ft-lbs.
Stating it was declined 'as .300 Win Mag ammunition falls above the 3000 Ft-lb limit which the HO guidance indicates in the table by omission of any calibres higher than this limit'.
The police refused to comment on the ballistic suitability and the overall good reason as 'that was not advised to them in the HO guidance and as such they will only consider it based on the ft-lb threshold'.
I personally will be ensuring I have appropriate legal insurance to cover taking this to court should my FEO/police force have an issue with the .300 Magnum, which I have held for several years now for culling large deer at extended range, at renewal.
Ben