Hi Sean - I was in the same situation as you but it's all sorted and the Garmin really is a great bit of kit!
Best thing to do is get the standard Edge 800 without a mapping bundle (I got mine for around £270). Then for maps, you need to go onto Amazon and buy a 2gb Micro SD Card with adapter for around £5. Make sure that you have an SD reader for your computer as well!
Then you can go to
http://talkytoaster.info/ and download the maps for FREE - it's based on Open Street Map and covers the whole of the UK. I've been using it for about a month now and it's fantastic - it doesn't allow postcode lookup but has loads of POIs and is routable. The instructions are all there on how to put it onto the SD Card. Then all you need to do is pop it into the Garmin and you're off
Planning routes - download Garmin Basecamp for free. Then from
http://talkytoaster.info/ again, download the basecamp version of the same map. This is a bit more fiddly but the instructions work fine. Then you install the map into basecamp and plot routes that you want to ride. Then it's just a case of plugging in the GPS, sending the route over and you've got the route with the option of turn-by-turn directions if you wish.
All very simple and no problems so far.
Going to do a video tutorial/review for my blog, will let you know when it's done
Hope that helps for now
Matt