personally i would say if you can follow a road atlas for long distance road planning you can read any map. the only difference finding different types of road.
With an OS map always remember; Red Main road = busy
Brown, B road = less busy but not necessarily so.
Yellow, minor road/country lanes = usually fairly quiet (but not always) but these are the roads to search out.
Decide where you are aiming for
weigh up a line between start point and end point
See what roads are running parallel to this route and pick the minor over the B and A roads. Approaching towns is less easy as often you have to go on to the more main roads for the last bit in to the the town.
Good advice to go out with a local group as they often know little routes and back roads that might not immediately be obvious on a map.
Remember modern OS Landrangers have various reccomended cycle routes marked inc sustrans routes.
Oh and keep of fthe blue bits as you might either get wet or find yourself staring down the juggernauts
Have fun I love map reading my way around the coutry and seeing the features on the map unfold in the landscape.
I stand and rejoice everytime I see a woman ride by on a wheel the picture of free, untrammeled womanhood. HG Wells