You need a legal reason for carrying, blade length must be under 10 cm (4 inch is just above that, so a smaller version could be a good idea). Fixed blades need to be carried out of reach. Automatic opening knives and butterfly knives are illegal. So are knives with "agressive" appearance (e. g. daggers, carambits, fantasy knives) or knives that appear to be other things (e. g. pen).