I live in Doncaster, South Yorkshire & I am with virginmedia, I've just been down to Kent on holiday & the mobile connection on my phone in Kent was really bad. I had to use my sisters AOL Internet connection. Now that I have returned home, I've set my phone back to Virginmobile; but I can't get a signal on my phone and/or connect to the Internet - all it says is that it a slow or disabled Internet connection.
I've deleted all the networks in APNs section of Mobile Network Settings & I've re-entered the giffgaff settings - still No Internet and I tried setting up a Wi-Fi settings and all it keeps coming up with is Remembered, secured with WPA/WPA2 PSK and then it Disables itself.
When the signal drops, switch to flight mode for a few seconds then switch back. That helps.
Try this, Go to Settings-Wireless and Networks- Mobile Networks-Network Operators- search for networks. A list of networks will soon be displayed. Select any network that is Not 02. (choose Orange/Vodafone etc). You will see "unable to connect" on the screen. Now select 02 and allow the selection back to Automatic.