It really depends on what sort of riding you do - road touring, or off road.
Best value at the moment is the Quest from GPSW: GPS Warehouse UK (Global Positioning Systems)
GPSW: Quest - and RAM Bike Mount (Buy/Full Details)
For £200 including mounting you can't go wrong.
I'd suggest trying one of these for a while and seeing how you get on. Once you get an idea of what these things can do, and what features are important to you, then you may think about changing it.
All of the Garmin models are good in their own way, depending on what you need it for.
The 2610/2720 are excellent for road touring and occasional off road use. The 2610 is a good buy if you find a new one on ebay - they're discontinued, but are well spec'ed and you can use cheap compact flash cards to store extra maps.
The new Zumo is easy to use, but some find it to be too simplified and lacking in features.
The 276c is very well regarded, but you have to pay extra for the maps (£150) and the Garmin memory cards are pricey, making them an expensive option if you don't really need the extensive track saving abilities.
The GPSMap 60 series are very rugged, and a good choice for off road use, but you need to pay extra for the maps.