You need to check this with someone legal but I think that when there is clear discrimination like this then you can wither go for the organisation OR the individual.
I know under HASAW you can do this, but your DD is not an employee.
If they are taking the 1-1 but not your DD then they have to provide her with a different 1-1.
From the government info:
Employers should not start from the assumption that all students with disabilities present a health and safety risk. A sensible approach to health and safety will help all parties work together to find solutions that enable access to educational opportunities.
So they may be in breach of the law, they are certainly at odds with the guidance.
www.hse.gov.uk/services/education/special-educational-needs.htm
Also
The public sector equality duty places positive obligations on educational establishments – the aim is to prevent problems from arising, rather than dealing with them after they have occurred, when it's often too late. Specific guidance for schools is available on the Equality and Human Right's Commission website.
Quote these at the head and say you will be informing the HSE of their breach. Remind the head that she/he is individually responsible and that as a minor your DD is entitled to legal aid.
I'd probably email them and tell them you are making a request uder teh data protection act.
You need the risk assessment for why your DD is being excluded from the trip - quote from HSE site, if they have not done a risk assessment then they are breaking DDA, Equality Act and HSAW act.
Ask what will happen if any child on a trip needs an ambulance? Your DD is no more likely to need one than any other child so if they have not risk assessed other children they are discriminating against your DD.
Then point out that under HASAWA
If a health and safety offence is committed with the consent or connivance of, or is attributable to any neglect on the part of, any director, manager, secretary or other similar officer of the organisation, then that person (as well as the organisation) can be prosecuted under section 37 of the Health and Safety at work act
Also ask for the risk assessment for your DD not attending the trip and the impact of removing her 1-1.
You need clarification of the role of the 1-1 on the trip, as they are not being paid by the school then are they there in a voluntary basis? If so have they informed who ever pays the 1-1 so they are not paid that day? If they are being paid then that's potentially fraud because the organisation paying (LA?) is not paying that person to attend a trip.
What are they providing to make up for the missed educational opportunity for your DD? You are legally responsible for educating your child, you have chosen to do this by her attending school, the school have to provide the same quality of education as others in the school.
I would basically cause them so much paperwork that it is less hassle to take DD on the trip.