I've heard that people usually use English lavender for ice-cream, but not French one. Any idea?
That is also what I've heard and read, and I'm not entirely sure if the article quoted in the other answer is specific to French lavender as it looks like a general article on both L. angustifolia (English) and L. dentata (French).
This article mentions that L. dentata is not usually used in cooking, although people have been known to use it nevertheless. This source also confirms this.
The wikipedia article on L. dentata says that it is commonly grown as an ornamental plant and its essential oil is used in perfumes. It is also used in Murcia (Spain) as a herbal remedy for stomach ache.
Here is a nice comparison between the English and French varieties. From the article (text in brackets and emphasis are mine),
While the other two [Spanish & French] aren't poisonous, it is the English variety of lavender that is most often used for culinary purposes, such as flavoring oils, butters and sugar, and in dishes, though some cooks like a little French lavender for a change of pace, such as in herbs de provence. French lavender can substitute for rosemary in breads and other recipes. English lavender is also most coveted for its scent in soaps, potpourris, essential oil, etc. The other varieties show up more often in dried flower arrangements