Just a quick heads up: I can confirm that the procedure for the APN settings in Android 4.2 are identical to those in Ice Cream Sandwich 4.0 and Jelly Bean 4.1, so you should have no problems setting up your Nexus 7, Nexus 4 or any other device you may have on Android 4.2 in the future.
The guide for the settings for the above versions is available here: http://community.giffgaff.com/t5/Learn-giffgaff-To
For a google nexus 4 running Android 4.2 (Jelly bean) you need to set up two new APN's one for internet and one for MMS
Go to Settings Choose More Choose Mobile network settings
Choose Access Point Names.
Press the Menu button on your phone and choose New APN
For your internet connection
Tap Name and type giffgaff-gprs then tap OK
Tap APN and type giffgaff.com then tap OK
Tap Username and type giffgaff
Tap Password and type password
Tap MMSC and type http://mmsc-ac.mms.o2.co.uk:10021/mmsc then tap OK
Tap MMS proxy and type 126.96.36.199 then tap OK
Tap MMS port and type 8080 then tap OK
You should see
MNC 10 if not then enter them manually by tapping, entering and OKing
Now press the Menu button on your phone again, and choose Save
Tap the little circle to the right of the profile named giffgaff-gprs on the list. It should turn blue
Now for MMS you need another APN
so go back through setting up a New APN as above then enter MMS Settings opening typing and OKing each
Name type: giffgaff - MMS
APN - Access Point Name type: giffgaff.com
MMS proxy: 188.8.131.52
MMS port: 8080
APN type: mms
go to your menu and save
To be sure these are set you can go back to the main settings menu and turn airplane mode on and off again.
You should now have two new APNs - Your phone is now set up for giffgaff's internet and MMS settings – have fun and if this works give this advice a Kudos!