A step-up is the board you'll turn to when the waves start to get a bit serious (6-8 ft+). Not called for all that often in England, but well worth having for those big days if you enjoy larger surf. And pretty much a necessity for that trip to Indo or anywhere else where you'd expect good-sized, top-to-bottom waves. Most step-ups incorporate a rounded-pin-tail and a narrow plan-shape for added control and drive in heavy situations. A bit of extra volume to help you catch waves is also a good idea. A gun would be yet longer/narrower, probably with a pin-tail, to provide even more control when the waves go XL.