"three is now owned by Orange and they will be sharing a network shortly – they'll begin automatically switching between networks based on signal strength shortly I believe. "
Not quite. Three is wholly owned Hutchinson Whampoa. Orange is owned by France Telecom.
T-Mobile and Orange are merging into "Everything Everywhere" and as part of the merger, their 2G networks will offer reciprocal free roaming. Orange already provides Three customers with free 2G roaming in places where Three don't have coverage. Seperately, Three and T-Mobile are merging their 3G networks.
Which is all pretty complicated but means that by early next year, Three customers are likely to be able to roam on to the combined Orange / T-Mobile 2G network and will use the combined Three and T-Mobile 3G network currently operated by Mobile Broadband Network Limited.