Three officially started to roll out 4G today to a select range of customers. They will continue to add more over time until everyone gets 4G enabled and just lives in a 4G area.
One step to allow 4G access on an iPhone is a carrier update which is a range of settings sent to your device and also to allow the option to toggle on and off 4G access.
Until now this was missing on iPhones using the Three UK network, however reports tonight are that this carrier update (15.6) is now being rolled out to customers on iOS devices and once accepted will indeed allow 4G on the device.
Please remember though this is just one step, you still need to have 4G enabled on your SIM which will take time. You will also obviously need to be in a 4G coverage area to use this service.
However one hurdle is now overcome when it comes to getting your iPhone closer to 4G on Three