Hi!
My dd2 (year 3) was born with her left hand missing from below the elbow.
So far she has not been on school action/school action plus. She is right handed, which I think does make a difference. Her infant school just made sure that if they were cutting then the TA was on hand (no pun intended) to help her if necessary. Most of the time she didn't want help, and stuck it under her little arm and cut her cardi/dress/coat with the thing she was cutting. 
When she started school I asked if she could do PE wearing her plimpsoles rather than taking her socks off and on as she found putting them on hard. She didn't want to be different, and so determinedly spent a day at home practicing, then could do it fine. I would make sure that on PE days she had simpler uniform for changing etc.
The junior school are thinking about putting her on School action, not to get medical hours, just that it's a big school and they want something to be on her records so that if either something comes up in the future it's quicker to get it recognised, and if they have a supply/new etc. teacher, it is there for them to easily see.
I asked the limb centre their view, and their view was that generally a child with a limb difficiency like this doesn't need any extra help, but if there's further problems (and being handed the side that's missing might count) then they might.
Are you a member of REACH? If so this might be worth asking on their fb message board. I know of one member whose daughter is missing the right hand (and is right handed), and I'm fairly certain she started school on school action or action plus. I don't think she got any extra hours though.
I have never regretted that she wasn't on the action/action plus register, if that helps.