You should have said something about the timetable last summer, not half-way through the academic year. Were you not asked to sign your allocation and make timetable requests?
I am a timetables, and I would have worked hard to avoid trapped time. For 14 teaching lessons, I would give you four PPA and one HOD lesson (so you would be paid for 19 lessons). I would put these on your timetable as "available", and all other non-teaching lesson as unavailable. In these unavailable times, I would not put you on cover or expect you to be in school and attend meetings. You would have 1 duty, and would have to do prep once a term.
If all your three days of working have you teaching 1st and last lessons, then you should have put your foot down. It is bad time tabling. It might be unavoidable because of the other members of staff in the same option blocks. If so, they should really be trying to find a way to fill in some of your trapped time, eg by teaching a couple of lessons in the lower school.
It is very difficult for the school to understand how you are feeling and then begin to resolve it if you are making it difficult to meet with your line manager. You might want to just think about using the hour of trapped time as this is less inconvenient to you than staying very late after school.
I think the reality of teaching an A-level only subject is that your teaching timetable is very part time, eg 8 hours a week. There's not a lot a school can do about this unless you are to teach a second subject or take on management responsibility - or really promote your subject so that you can have two classes.
Have you had a look at the school timetable to see if your students have study periods in the middle of the day, and then switch the lessons yourself? Sometimes it is very easy to break option blocks like this, especially if you have a small number of students.
As for targets, I suggest you write your own. The targets have to be relevant to and achievable by you, so there is no point in your having anything that is only applicable in the lower school.
It is a little bit worrying that you have a large number of managers newly arrived from the state sector, but they will have a lot to offer your school. However, they need to be trained too by you - it works both ways. Keep reminding them that it is all about the individual student.