Samsung phones have an error/bug when installing the APN.
Recheck the APN, it's easy to miss the error
The line for MMSC reads MMSC: http://mmsc.mms.mediamessaging.co.uk:8002 with an extra mms installed
It should read MMSC: http://mmsc.mediamessaging.co.uk:8002
You're unlikely to be able to edit the current APN
Create a new APN, call it giffgaff2 if you like, with the settings below
Connection Name: giffgaff
Proxy: Not set
Port: Not Set
Password: [leave blank]
Server: Not set
MMS proxy: 22.214.171.124
MMS port: 8080
Save the new APN
Turn OFF mobile data, select the new APN and set as default.
Reboot your phone, turn on mobile data.
Make sure wifi is OFF when sending MMS messages