at what score will the teachers accept as a borderline score
There's no such score. Schools will decide in each individual case whether - and how strongly - they can support an appeal.
also the head does not know the child as well as the form teachers so will they also contribute to a appeal letter if needed.
One would hope that the headteacher and class teacher actually talk to one another - it's what happens in most schools!
- and that information from the class teacher will influence anything the headteacher writes.
In most cases the only statement of support is from the headteacher. It might be possible for the class teacher to write a separate letter, but it depends whether policy within the school permits this.