RootMetrics shows a continued lead for EE (previously Orange and T-Mobile), but the gap is narrowing. Interestingly, each time these surveys show Vodafone at the bottom, Vodafone says their "customers are experiencing a significantly improving network". Still leaves Vodafone significantly behind the other operators.
There is a plethora of mobile service providers and resellers (referred to as virtual mobile network operator or MVNO). The MVNO performance should largely follow the performance of the host network, though there can be subtle differences depending on how the MVNO is run. If you want to know which mobile network your MVNO service provider is running on, check out the UK MVNO list on Wikipedia.