Im a nanny daily and eat all my meals with the children when on duty. So in term time i eat lunch and dinner, and in the holidays i eat breakfast, lunch and dinner with them. I ALWAYS eat exactly what the children eat, but then they are good eaters and eat very 'grown up' food. (I refuse to feed "kids crap" as i call it)
I do the food shopping for me and the kids once a week and the mum refunds my expenses (tho in the new year we plan to share an online ocado order and she will top up from M&S)
Im quite fussy in the meat i eat (i was veggy for 7 years) and will ONLY eat chicken that is AT LEAST free range (normally organic) and the same with eggs etc and i refuse battery pork (i pork made ourside the uk.
Luckily my boss and i share the same opinions (tho she is actually veggy) and she will only purchase organic meat etc so it isnt a problem.
IF i were to go veggy (which ive been seriously thinking of doing tho at the moment decided against) i would not expect her to provide extra veggy stuff for me, tho the reality would be im sure that the children would get much more of a vegetarian diet.... no ham on (home made) pizza or bacon in pasta veg and cheese bake etc.
Also the one and only thing my boss buys extra for me is lady grey tea and real organic butter (cause i wont use anything else to cook!)
If your nanny is a live in then she should be able to eat all the meals shes normally eat, but i think daily nannies should only 'expect' lunch, in which case if you have bread, jacket potatoes, cheese, eggs, beans, salad etc that should be fine!
Also i wouldnt think it would be unreasonable for you to get meat sauages and veggy sausages for a veggy nanny for example, so she can eat sausages with the kids, tho if they are having lamb chops then maybe its best if nanny eats at home! maybe also a batch of veggy mince so if they are having spag bol / pasta bake etc nanny can have the same in veggy..... she could make a big batch and freeze it in single portions!
As for petrol 40p per mile is the norm... you basicaly pay your nanny her salary tax and NI through nanny tax (or howver you want to do it) and give her the cash / cheque at the end of week / month to cover expenses including petrol. Given thats it is essentially reimbursing expenses there is no tax to worry about on it.