Not sure if it's some sort of o2 policy but they are definately telling porkies when you try to leave.
I ported over my main phone at the begining of the month and was asked if I realised that gg runs at 1/2 power compared to o2.
O2 Rep : "if you have 4 bars with o2 , you will only get 2 bars with gg"
I've subsequently heard a few more good 'uns.
I can understand them not wanting lose customers but out and out porkies is a bit naughty !