turn your phone off then on again after a few seconds, it may be using a mast that is damaged or too far away, it will pick up the closest t mobile or orange one after u switch it back on
I have been having the same problem over the past few days though I'm in Manchester.
I thought the network may be being hammered somehow with the iphone software upgrade/new iphone being out. Not sure if that could make a difference though.
I googled to see if I could find anything about a service issue but couldn't find anything. Worth checking in your case though.
turn your phone off then on again after a few seconds, it may be using a mast that is damaged or too far away, it will pick up the closest t mobile or orange one after u switch it back on