I don't think you can get them from OS but Harvey Maps (
www.harveymaps.co.uk) do produce contour only maps as part of their custom mapping service - unfortunately no idea of the cost. Harvey don't produce maps for the whole of the UK and I would have thought, but I'm by no means certain, that they only provide contour only maps for the areas that they produce full maps for.
At the back of my mind is the possibility that I read on a forum somewhere, or equally I could have dreamed it, that someone had scanned a section of an OS map and then used their Photoshop skills to remove all colours but brown so producing some sort of a contour only map but, if it were indeed possible, I would have thought it must be far from perfect as the contours would have a gap in them where e.g. a stream crossed.
I also believe the Talkytoaster company (
www.talkytoaster.co.uk) that produce maps for Garmin GPSs somehow add OS contour information to an existing open source map so maybe they can produce some sort of contour only map.
Hope this helps.