Telefonica own O2 and GiffGaff, so this is nothing new for them
GiffGaff and O2 are both owned by Telefonica, and GiffGaff runs on O2, so they've had their toes in the water for ages here. The market changes (and what O2 is currently offering to customers) reflects that. It will be interesting to see how both O2 and GiffGaff develop their offer.
Don't know if other major telco-owned MVNs like GiffGaff are running in the UK, but I wouldn't be surprised to see others popping up to try and capture that growing part of the market whilst the mainstream brands are used to service the customers who are willing to pay premium prices and/or want premium services.