Thanks, guys, but more questions than answers
Originally I tried to send it with wifi on (at home) and whilst I did not actually try send it while outwith my wifi range, I would have expected the phone to send at the first available opportunity wherever. I have also tried to send with my wifi off. I sent it to my wife's android phone and I wouldn't expect hers has any file size limit.
The photo file was taken internally by the phone camera, so I downloaded it to my PC and can see it is 2.12mB. That may well be large but why then would an iphone take a photo which it can't send despite having an icon to do it? Other android phones seem to be able to do it no problem.
All phone numbers in the UK have their 'area code' included as standard.