In reply to cousin nick:
+1 for Edge 200. Less than £90 and will do exactly what you want. Though only if you can plot your route online at home first as stated, then upload to device. I use mine on MTB and road and means I never worry about which turn to take. I also enjoy plotting my routes first at my desk instead of working then going out and riding them.
It's always funny on threads like this when people respond with 'been using a map for years its fine'. Of course it is, but it's 2014 now.
Pre-internet, conversation in the pub. Q. What map is best for going out riding? A. You don't need these new fangled map things boyo, I've been using a Sextant and toggle rope for years and works just fine.
Wow, I've gone all UKC
Enjoy the road, hope it helps!