When down gets wet, it will lose most of its insulation properties but this is where Hydrophobic down comes in - this is a relatively new concept and the purpose is to solve the main drawback of 'regular' down, which is it's lack of effectiveness when wet.
Hydrophobic down has been treated with a durable water repellent that enables the down to dry quicker and resist water for longer, meaning it will perform better in damp conditions. This makes down jackets that utilise hydrophobic down more versatile, as they can be used in cold, damp conditions without being damaged, which is great for the British climate.
Jackets using hydrophobic down tend to be slightly more expensive, as it is another process and expense for the manufacturer. Whether or not you need hydrophobic down depends on personal preference and whether you feel this is a feature you need, although more and more manufacturers are using hydrophobic down in their products.
Advantages over regular down: