Moved to Plymouth England last Feb (2010). We looked online for couple of months before moving over to get an idea of what was available, etc, just as your doing.
Once arriving, we called letting agents, but quickly learned it was better to get on RIGHTMOVE and FINDAPROPERTY websites to find a place. We searched both sites several times a day, when we would find a place of interest, we then called the agent and made appt to view. There were one or two other agents that we found online who did not use RightMove or Findaproperty to list their rentals, so you might want to do lots of different searches and see who isn't on the 2 main big sites.
We also found the just riding the areas we were interested in was another great way to find rentals.
We found it to be SO TRUE, PICS DO NOT TELL THE REAL STORY about a place, the rooms, etc. We looked at many that looked awesome/great online, but when we were actually there viewing we were shocked at how different it really looked.
There are plenty rentals everywhere I believe, just keep doing what your doing, then after you arrive, take it one day at a time until you find the one you really want. It took us about 2 and half months to find the 'right place for us', the one we really really liked, and we have been happy living here since then.
Nemie