Well they can always say "accept or we'll give you notice" and that's completely legal. But this is usually a bluff, if they want you bad enough to want to increase your notice, they're not gonna let you go for merely negotiating.
Companies often take existing employees for granted and act tough, but once you don't cave they know it's far more expensive for them to replace you than to give you what you want. So they eventually cave. That's why I always advise people to interview for new roles regularly to be aware of their market value and be confident that they can easily find another job, so they'll be confident if/when these negotiations come up with their employers.