Ahh right got it mate.
Then it gets confusing.The next paragraph says.
Pay Monthly & Pay As You Go customers
If you’re on a Pay Monthly or Pay As You Go tariff, you can ask us to unlock your device at any time, as long as you’re in the UK. We can only unlock your device if you bought it directly from us, otherwise you’ll need to contact your provider.
As we all know giffgaff don't lock phones,and there is no mention on the o2 page it will cost £15 to unlock the phone.