you can book online. I do it all the time. From the page that "justinc848" posted above, click "go to search" and it will bring you to the page.
It will have "Search for hotels or take advantage of exclusive VIP rewards." on it.
alc
It may depend upon the specific hotel, or their room inventory, because I just tried to follow this process online and when I got to the page offering hotels to select it only showed me a little icon of a telephone, and asked me to call a 1-800 number to make the booking - there was no option to complete the reservation online.
This is presumably geared toward the North American market, so I am unsure if the UK based Hilton line can assist with this -- any clues ?