Some vendors are better about it than others. I've had a bad experience with O2 denying my guarantee rights under the Sales of Goods Act, for what was clearly an "unfit for purpose" phone (iPhone 5 with a faulty sleep button[0]). Apple replaced it for me of their own accord, though.
[0] - https://www.apple.com/uk/support/iphone5-sleepwakebutton/