Some parts of Berkshire (the districts of Bracknell Forest, Slough and Windsor, and Maidenhead) are in the "fringe area". The rest aren't.
So in a maintained school after 6 years (assuming that you hadn't yet been assessed as meeting the Threshold standards) you would be on £42,689 (fringe) or £41,334 (rest of England). NB those figures are the current ones. They will go up in September, but it's not clear by how much yet.
Your school may be paying slightly above the STPCD rates, on the grounds that it wants to attract good teachers, or may be paying slightly less, on the grounds that it can still attract good staff because the working conditions are better/less stressful.
But as @ThanksItHasPockets has said, you need to consider the salary in the context of the other conditions/perks: pension, opportunities for progression and discounts on fees. But also things like class sizes, pupil behaviour and working environment more generally.
The school might be willing to negotiate, but they will need to fit you somewhere into their pay structure. I imagine it will depend on how keen they are to have you and how easy they find it to recruit and retain their teachers more generally.
I wouldn't expect them to take extra-curricular activities into account. I imagine the vast majority of teachers in independent schools are expected to do some extra-curricula activities. And the travel time is your problem not theirs (and presumably one that you were aware of, and happy with, when you applied for the job).