The hill probably isn't that bad really. What makes it hard is that it comes very near the end of the race when your legs are already tiring and you've been running for mile after mile at a steady pace on straight, flat ground. The hill is then a "shock to the system", mentally probably as much as physically.
You run down it soon after the start, so you get some idea of what's in store !
Nevertheless, it's still a quick course and you can aim for a time considerably faster than for example, the Deal or Canterbury halfs which are much hillier overall.