Here’s an interesting one, the guys over at OpenStreetMap now have a map that is compatible with the Garmin Edge 605 and 705 (and other non-cycling specific Garmin nav units). I won’t replicate all the instructions here, simply head over to the download page and either download the ready made map (link near the top of the page) or follow the instructions to make your own map.
Since the recent Garmin Edge 605 and 705 firmware update it’s now possible for multiple maps to co-exist in the same storage area, for instance you can have City Navigator and Topo maps on the storage card. Previously you had to put one map on the internal memory and the other on the Micro SD card. So if you now put your map(s) on the storage card, you can then put the gmapsupp.img file from the link on this page onto your internal memory. I couldn’t figure a way of putting the open map in the same memory area as my other maps as you have to call it ‘gmapsupp.img’ which would clash with the already existing one.
The UK map has routes from the National Cycle Network on there, but unfortunately autorouting is not possible, but then just how much do you want when it’s free 
Not a bad solution for someone who is having to live with the free base map until funds allow an upgrade.