Poor you! This is just appalling! My guess is they understand very well, but are trying to wear you down. Did you manage to speak to the solicitor before going into the meeting?
I would call ICO and ask how to enforce the regulations. There is no point to these rules, if there is no way to enforce them.
Can you get the police to get the answers you want? How is the police supposed to assess the risk, if they don't know what information has been passed on? Once the police has this information, they should tell you. I my (very limited) experience, the police can be extremely helpful in these matters.
Contact your local authority and make them aware of the safe guarding issue, severe data breach and the college's refusal to cooperate with you. Ask them to interfere and to make sure the college acts lawfully. Try to communicate in writing with them as much as possible. They might side with the college though, even if you are in the right (I have experienced this once).
Check for the complaint procedures of the school and make official complaints about the SENCO, Vice principal, principal, head of safeguarding and anyone else involved to the governing body. List everything that has happened and what you expect to happen with deadlines.
For any further meetings, take a witness with you and ask for the meeting to be recorded.
Contact your MP and the DoE. Copy each relevant rule and regulation and then very briefly state how they have been breached and ask for their support.
Did anyone take minutes at today's meeting? If yes, ask for them. If no, write minutes and ask them to make changes by Monday, otherwise you will take the minutes as accepted as is. Ask for the updated safeguarding guidelines to be send to you by tomorrow.