I'm with Voxi and have zero complaints about the network - it's just Vodafone (right down to the network ID on the phone being "vodafone UK") with better pricing.
My only criticism is that they have zero customer service. The live chat doesn't get you through to anyone (I once left it waiting in a tab for several hours with no response) and the recommendation is to "use Twitter for customer support". I'm not much of a social media user and don't have a Twitter account.
For a basic mobile phone service, though, there isn't that much need for customer service agents. Very little can go wrong, but if something does go wrong and it's your account rather than a network-wide issue, good luck without moaning on Twitter.