How likely would it be that if I buy cycle wings from Dax in white that they will match white panels from MK? Would I need one manufacturer or the other to match the RAL code?

Both the cycle wings and the MK panels are probably gel coat and therefore likely to be slightly different - you may be lucky and find they are close enough in colour that being mounted separately from each other it won't show.

Much like paint, there are a zillion shades of white so the chances are slim.