Have a look at this website:
http://www.pocketgpsworld.com/
Lots of reviews and information about available satnavs.
I would suggest that you find a couple of likely candidates from there and then go and play with them in Halfords or Currys. The only way to find out which is easiest for you is to play with them. I had an old TomTom and decided to replace it. I chose a Garmin because of the battery life (I wanted to carry it round foreign cities). I find the Garmin nowhere near as easy for me to use but other people prefer them; you can only find out which suits you best by playing.