Recently switched to giffgaff myself.
£10 goodybag for the month gave gives me 250 mins, unlimited texts/data and free 0800/0500 numbers.
….and to quote myself from another post:
I am in the process of switching to GiffGaff from O2, and keeping the same number, using a 3GS.
Gone from £20 for Simplicity sim only for 600 mins / 1000 txts / Unlimited Web (which while decent was waaaay to many mins)
to….
£10 goody bag on GiffGaff which gives me 250mins / Unlimited Web & Texts / Free 0800/0500 calls.
If all goes to plan it would have gone:
Wednesday: Apply for GiffGaff sim & ring 02 for PAC Code.
Thursday: Receive, activate sim and put in PORT request on the GiffGaff website. Also download My GiffGaff off the Appstore.
Friday (hopefully*): Number gets ported over and relax.
Probably mentioned above, but both GiffGaff and Tesco piggyback on 02’s network.
* All went smoothly.