Another recommendation for bikehike.co.uk
Having played around with a few different sites, I keep coming back to this one. Integrated OS mapping, a darned accurate elevation figure, simple to plot routes, simple to get them onto the garmin. I can think of a route and be out of the door with it on my Garmin within 10 minutes (I have done this).
The only alternative I've been using recently is to plot out on Google Maps, then put the link to the route through gps visualiser to convert it to a gpx which I can then download. Gives a track for each leg, but this can be stitched together either manually (which I do) or another package (which someone might be able to recommend). A bit more faff, but is super handy for DIY by GPS if you are an AUK.