Interesting question.
The warranty supplied by the manufacturer/supplier is IN ADDITION to your rights as a consumer, which effectively say that goods must be fit for purpose.
Now for example a TV would be expected to last a lot longer than 12 months warranty, and as such you can go back to the store and say fix it please - for free.
This is one of the reasons why car manufacturers now give 3/5 year warranty on their products.
So i guess the question is, how long would it be reasonable for a mobile phone costing £300 to be expected to last? My guess is 2/3 years.