Actually, it isn't. The retailer is responsible for facilitiating repair for inherant faults. For the first 6 months it's assumed that the fault is inherant, after that it's for the customer to prove the fault was inherant and not developed (sales of goods act). Something that's rather hard to prove with a mobile phone. Most retailers will assist you in sending the phone to the manufacturer (or someone authorised to do warranty repairs on their behalf) at any time though.
Don't confuse warranty with statutory rights, the two are fundamentally different things. Your statutory rights are with the retailer, your warranty is with the manufacturer. The warranty gives you additional rights and benefits to your statutory rights. A warranty protects you against faults developing for the length of the warranty, whereas your statutory rights protect you against inherant (ie present at time of purchase) faults, with restrictions and limits.