anyhting that's non-standard counts as a modification, so, technically, yes you do, otherwise the small print may invalidate your policy.
No idea about premium increase I'm afraid.
i hope you havent tinted the front windows,as its against the law and your insurance co,could and would refuse to pay out on a accident,they will use any excuse.
Whatever you do to modify a car you should run it past your insurance company. I you make a claim they will find any little reason not to pay up. Don't give them ammunition.
Yes, its classed as 'making the car more desirable to break into' so it does need to be declared, and yes the front windows are illegal if you can't see though them by a certain %age, .