No depending on your property if you do not have a lawn or plants then you probably will not need Brown bin liners. Depositing Compostable bags of kitchen waste only into your Brown bin ensures it does not become dirty from food material sticking to its inner surface. However if your household uses a lot of fresh vegetables especially Cabbage, then using a Brown bin liner allows you to deposit large leafy items directly into your bin.
If you do have a garden then buying just 6 months supply of liners for your Spring to Autumn garden waste along with your Kitchen Caddy Liners will keep your Brown bin free from Green waste sticking to it, or worse still Composting in it and stinking to high heaven. If the budget stretches buying 12 months supply of liners will give your bin all year round protection.