Actually, that isn’t a given. While a MVNO has access to the same infrastructure as their parent network, they don’t always get unfettered access to all the bandwidth, speed and capacity. 

This is true for all MVNOs but giffgaff in particular has had challeneges in the past as their data usage far exceeded the average O2 user, straining the resources that were available. That seems to be behind them now so while the service may not compeletely match O2, you can be confident it is still fit for purpose.