To keep the guns safe while I am there, I want to install a gun cabinet. Do I have to tell the police about this arrangement?
No. So long as the Midlands address remains your main address there is no need to inform the police.
In your circumstances, especially if you have a rifle, it is very sensible to install a cabinet in the place where you regularly use your guns.
Your relative may have a shotgun certificate so there?s no harm in letting him have access to shotguns.
He?s unlikely to have a firearm certificate for your rifle (even though he may have one for a rifle of his own) so he must be denied access to it as far as is reasonably practical.
The same considerations as to choice of cabinet, siting etc., apply to the temporary storage as they do to your storage at home.