As I see it. It is a question of branding and language. If Nestles trade mark it, presumably Mars, Cadbury's or whoever cannot subsequently use the phrase in association with their own products.
The KitKat is certainly a powerful brand and as much as anything else in the British public conscience this must rank high amongst iconic symbols and hosehold names, like say, Hoover and Sellotape. But does mean that ordinary words associated with the brand automaticaly assume the same status? I think not.
It reminds me of the Kellogs slogan, 'The best to you'. I assume this was trade marked. However this has a rather odd sentence structure and isn't likely to be used in every day speech. Conversely, I would say that 'Have a break' is too common in linguistic senses to be trade marked and I hope Nestles lose because they've wrecked a fair chunk of my favourite chocolate and don't deserve to win.