I can't declare the value because Samsung have provided the prepaid postage label. Wouldn't Samsung be liable for £600, as they have organised the return, the carrier is irrelevant?

Hi, I'm upgrading my phone through Samsung's upgrade programme, they said it was free to send the old phone back. When I received the envelope to send my old phone back, the leaflet says I'm only covered up to £100 if I use their prepaid Royal Mail tracked label. They recommend buying my own Special Delivery label if I want higher coverage for loss/damage. Am I right that under the Consumer Rights Act, they would be liable for the full value of my old phone (£600) for loss/damage if I use their postage label, and they can't limit their liability by stating an insurance limit of £100 with Royal Mail? I would appreciate any advice, thanks.