MM Tracker[/url]
Memory Map OS map tiles, GPS position and track/ route overlay on your phone.
If you are worried about battery life, turn the phone onto ‘Aeroplane’ mode (ensure GPS is active though) and your phone should be good for all but the very longest of rides. My HTC HD has survived a 6+ hour ride with MM tracker on for both route finding and recording.
EDIT:
Also look into GPSIES app.
I generally use wheres the path website to create routes using their full screen OS maps. I then upload the route for storage and sharing purposes on the GPSIES website. If you have this app on your phone, you can access all of your routes or browse for routes nearby when you are out and about.
You can then download a route that takes your fancy direct to your phone through the app and open in in MM Tracker so you can follow this on via OS mapping.
Seems a bit drawn out when written like that ^ but is very simple to actually do.