Yes, please do write to your MP explaining as succinctly as you can what the problem is and what your local authority has said and what you'd like your MP to do (take up cudgels on your behalf?)
I gather that invitations went out yesterday to all MPs for the All Party Parliamentary Group meeting. Minister Nick Gibb will probably be there. I think that if MPs get a personal request from a constituent or two they really don't have much of an excuse for not attending a short meeting just down the corridor from where they'd be hanging out anyway. (They're all in London on Tuesdays.)
I've just updated the exams page on my website with more info about Access Arrangements for the actual exams and for controlled assessments. edyourself.org/articles/exams.php
The better-informed your MP is - especially if he knows that other areas (like mine in Sheffield...) are doing more than your council - the less he'll be fobbed off by the council saying they aren't allowed or whatever.