If you can't send an MMS you might have the wrong Access Point Name (APN) settings. They should be set automatically, but occasionally you'll need to pop them in manually. It's a doddle!
On Apple devices, you can find the settings within the Settings app. In the Settings, go to Cellular (or Mobile Data), then Cellular Data Network.
On Android devices, they can usually be found in the Settings app under Mobile Networks/Networks, Mobile Network, then Access Point Names/APNs. Make sure to tap the new button (it might be hidden in a menu behind three dots).
MMS APN details
APN: giffgaff.com
Username: {Leave Blank}
Password: {Leave Blank}
Proxy: {Leave Blank}
MMS proxy: {Leave Blank}
MMS port: {Leave Blank}
MCC: 234
MNC: 10
Authentication type: {Leave Blank}
APN Type: mms
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