As i've said on other similar threads before, their existing tariffs seem to be selling well, and if new customers are actively signing up at the current prices why would they change?. Its not an astute business decision to begin offering "more product for less money" if people are already happily signing up at your current rates, an action which promotes full silent acceptance of what you are already giving them.
The only time anything will change is when they start losing large chunks of customers, and I can't see that happening soon, at least not whilst their social network pages are full of people who think that £27.99 / 5GB really is a great deal and don't seem phased by the fact a few months ago they could have got 10GB for the same price on the same tariff
Let's face it, there isn't exactly a lot of toe to toe competition is there, at least not if you want 3G / 4G outside of a Town Centre.
Vodafone do seem to be making progress, after all it seems they will be the first to bring 4G to the Town where I work, however where I live, Vodafone don't even have 3G for about 6 miles, so personally i'd class their roll out as sporadic in some areas, non existent in others