Been with Virgin Mobile for 3 years now
Unlimited calls, texts and 6gb data which rolls over. £8.20/month. I think I've had down time twice in those 3 years. I can't really complain about their service, especially for the price.
I'm happy to sit on this plan forever as it goes up by inflation, or to grab another one whenever they next have an appealing deal. I rarely use the 6gb though.