Call cs on 202 and choose the "thinking of leaving" option.
They will offer you a £4 a month (approx) discount for 12 month sim only deals, but, it still won't be as cheap as giffgaff.
There are trade off's with gg though like data speeds and reduced bandwith, so if you want best performance, stick with o2, even if it's £2 a month more expensive.
You can also get better data allowances on o2.
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