We have done this in the past, I had all the ferry companies uk phone numbers (the regional not the 0845 ones) when we were thinking of coming home we went up to Cap Gris Nez as we knew we could get a O2 mobile signal there & I just rang around the different ferries to see what the best deal was. Some want you to give a specific date/time but we found Sea France the best to use. I asked for the cheapest fare back to UK in the next week & in 2007 it cost us £30 one way for the next day.
If you know when you intend to return it is still probably cheaper to book in advance. We tend to use eurotunnel & paying with Tesco clubcard vouchers, that way it only costs a quarter of the full price & if you book far enough in advance you get the cheapest rate.