Depends on how much your going to be using it tbh?
I sell sat nav for a living btw.
Garmin I3 for the money is very good.
Tom tom One Uk is easiest unit to use on the market. Currently £199.

Navman F20 with full post code search and 3.5" touch screen is very good for the money £150.

Any of those depending on how technical minded you are and how much your be using it.