In the eyes of the law, you’re liable for a speeding offence as soon as you exceed the limit - even if it's just by 1mph. Guidance from the National Police Chiefs’ Council (NPCC) does recommend giving drivers a so-called ‘10% plus 2’ leeway, to aid police officers in using discretion, however, it's worth remembering this is only a recommendation, not the law. I would predict 35 in a 30 will be a penalty but you'll have to wait and see.