Money no option: Edge 705
You may not need the maps but they're nice to have, it's a lovely little unit. The free routable OSM maps work well. Can be found for under £300.
Within budget: Edge 500
It is a lovely little thing, but it's really comes into its own with a Powermeter (SRM, PowerTap, Ergomo), without one it doesn't give you much more than the Edge 305 except for things being a bit easier to use (nicer screen, much larger internal memory, data files transferred directly as it appears as an HDD, etc).
Cheap: Secondhand Edge 305
Lots available on eBay as people upgrade to 500/705/other. Same ability to upload a route and have it point you in the right direction with an arrow, but less expensive. Same ability to record speed, position, cadence and HR.
For all of them remember that you get one GSC10 (speed/cadence sensor) with the GPS. Each extra bike you want to use it on will cost at least £24 or lots of time shifting them from one bike to another.