I have an iPhone 6S which has developed a battery issue. Apple have now agreed to replace my device but are only going to replace with a “service device” / “as new device”

This seems very wrong to me as under the consumer act I should be getting like for like? I did not purchase a second hand refurbished iPhone so I should be offered one as replacement?

Has anyone tested this case with Apple and if so do you have any pointers on how to proceed?