No it isn't. Not remotely.
There is no real obligation in law to slip a firearm or a shotgun.
I agree.
It is an offence to have loaded shotgun in a public place (which includes your car) or ... any firearm without good reason.
Actually, it is 'any other firearm (whether loaded or not) together with ammunition suitable for use in that firearm' and the good reason is (unsurprisingly) that I'm out foxing. Loaded or unloaded makes no difference under the Firearms Act.
In the eyes of the law, having rounds in the magazine, whether its in the rifle or in your glove box for example means that the firearm is loaded.
The rifle is 'loaded' if the charged magazine is in it. If the rifle is free from rounds, it is not loaded. Not that it makes any difference under the Firearms Act (see above).
You can have the gun unslipped but not rounds in the mag or gun.
Actually, within the bounds of common sense you can do what you like. When I had a foxing rifle with a fixed magazine, I used just to put the rounds below when driving between beats. The Firearms Act does not recognise the difference in a rifle between a charged magazine and a chambered round: I do, though!
I approve entirely of people taking reasonable precautions at all times. The whole rigmarole of all shooting gear out of sight, bolts out, ammunition hidden and so on is exactly what I do when transporting rifles and guns from home to shooting-grounds. These journeys may go through towns, may be in daylight, might involve stopping for fuel and so on. So be it.
However, when driving a vehicle off one field, down a road a wee bit and then onto another one in the middle of nowhere in the dark when I am either in the vehicle or within 5yds of the locked vehicle, I think my 'precautions' are entirely reasonable.
We must be grateful that we have been granted the freedom under the Firearms Acts and Police Security Guidance to make up our own minds on these matters, knowing that we might well have to justify out actions to the Police or Courts at some point. I do sometime worry that excess zeal in some might be used to raise the bar of 'reasonable', but I guess that's life.
As an aside, I'm happy to wear (discretely) a knife when I have a reasonable excuse, such as I'm on my way to shoot bunnies or stalk deer. I suppose I could leave it in the vehicle, but then, just like the bolt which I also carry under such circumstances, it might get into the wrong hands. Just a thought.