Are you talking about the 120g one or the small one?
The size slash happened 3 years ago, with just 20g being taken away. I'd rather that than a price increase to be honest. On the bright side, that's less granulated sugar, trans fat, and calories consumed if you decide to indulge in one of these things.
These go for a £1 in most shops, for a big slab of chocolate, which is a bargain to be honest. Higher end chocolate is far more expensive and smaller, such as the Lindt 100g bar which is nearly double the price.