Well so long as you're not reliant on them for a good reference then in most cases they'd probably not do much. I guess technically you might end up being officially sacked/terminated though if they're not happy with you just deciding to not work your notice/not turn up. After all they're not going to want to pay you for that period of time and if they didn't agree to it then simply not turning up at work would be grounds for a disciplinary AFAIK.