Jord, as long as the tyres are in spec then no you don't need to declare them. As an example I think you'll find however if you fit tyres of a lower speed rating from OEM you'll have invalidated your insurance.
Winter tyres however you should declare them, but they should not increase your premium. Not all mods increase your premiums.
It boils down to whether the insurance company decides to cover you if you haven't declared mods. If the car gets stolen, they recover it and it has something on it that makes it stand out from another car in effect making it more attractive to a thief, be it wheels, a wing, tints or wind deflectors, they'll use it as a reason not to pay out.
With regards to whether it makes you chunt, if I get hit by someone with undeclared mods and their insurance refuses to pay out stating that they invalidated their insurance, then yes they are a chunt.