@ 0None0 the child has a 1-1 who is medically trained. If the child can’t go on that trip, then the school needs to arrange one they can go on, or no-one goes.
Added to which, the school have given 4 different reasons why this child can’t go, so clearly it’s not really about the child’s needs but about their own need to discriminate against a child with disabilities.
OP first and foremost I would be telling the school that if they refuse to take your child on the trip then she will be attending school as normal so obviously her 1-1 will be there and not on the trip with the other children. Lack of ratios in that event is their problem to deal with, not yours and not your child’s, your child will be in school, as per their request.
Secondly I would speak to the chair of governors tomorrow and arrange a meeting ASAP with them, the head, and the senco. I would state that the desired outcome of this meeting will be that either your child attends the school trip, or there is no school trip at all.
Also, do you know where they are going? If it’s a particular venue I would contact them in advance of the meeting with head etc to enquire about their provision for children with disabilities, and I would ask if they would exclude a child with disabilities from their premises. If not, I would mention to them that one of their school parties is intending to do just that, and that in the event you go to the press they will likely be approached for comment.
No venue which receives payment is going to want to be named and shamed as a venue where the disabled are not allowed, so if they are prepared to speak out about the fact a child with disabilities has been disallowed from attending their venue by a school, that is going to look worse on the school. And if they agree the disabled child isn’t permitted in their venue or they have no provision, it is going to affect their business.