The Sussex website says...
http://www.sussex.ac.uk/physics/ugstudy/2015/31103#qualificationsUnfortunately I guess they mean unable to do the A levels because the 6th form doesn't offer them at all rather than unable because the school thinks you're not good enough at maths.
Could you redo the GCSE maths alongside your other AS subjects and then do AS maths along side your A2s?
The suggested OU unit or even picking up some relevant (i.e. mathsy)
MOOCs would probably strengthen your application IMO
It's going to be very Maths intensive and I think they'll probably want to see *some evidence* of recent study & success in Maths on your admission.