I had that problem with the iphone 4s on another network before. There's nothing really much you can do about it because chances are you're going into a blackspot for signal.
What's the coverage like in your area?
Do you know anybody else on 48 or O2 and how do they get on?
If OK then can you try their SIM in your phone and your SIM in theirs?
Hope this helps.
Google it (e.g. "iphone goes from 3G to E") and you will see that this is not unique to 48.
Hope this helps.
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