I’ll gather some factsheets from health visitors and sit down with them both and explain the importance of the condition and what to look for etc.
This probably won't work. As a PP said you are better of telling them what she can eat.
Also simplify it for them so instead of saying she can have dairy free butter (which is actually spread/margarine) and dairy free yoghurts, simply say she eats fresh bread on it's own and doesn't eat yoghurt.
Just worried that they won’t take my input seriously as we have had arguments in the past over using formula as I was originally breastfeeding.
Unfortunately it is likely for a few reasons. There are people who don't take their own allergy seriously let alone that of another person. They don't realise allergies can kill or cause serious injury. There are also people who pretend they have an allergy or intolerance when there is nothing wrong with them, and there are people who pretend an intolerance is an allergy.
Easiest thing to do is to send food with her that she can eat. Though only send processed food with her that you can see only has up to 3 ingredients and send unprocessed stuff which they may have to mash up with a fork before feeding it to her so they can see she can eat fruit, veg, potatoes etc.
Lots of adults with allergies take their own food with them when out or you simply don't eat if they are unsure but the latter isn't an option for a child.