I’d like to map out an offroad route using OS software online, then email the link to a mate. Ideally a website like Bikehike – but one that allows me to save routes and then email a link, not just download to a gps. Do such exist? Bikeroutetoaster is the one I use for road rides, but doesn’t have the OS side of things.

Mark out the route on bikehike, save it as a .tcx file, email file to mate, mate can then upload the file to bikehike.

This should do you:
http://www.getamap.ordnancesurveyleisure.co.uk

I tried: http://www.trailzilla.com/ the other day, I was really impressed with it, I believe it allows you to create a link to routes (though they may need to sign up).

Not sure if it fit your bill, but was trying to create gpxes from the bought brecon beacons maps packs.

You can map a route on Bikehike, then save it as GPX or TCX, then upload it your own webspace.
Then you can get a URL for Bikehike to load it from your webspace.

eg something like this: http://www.bikehike.co.uk/mapview.php?lnk=http://craig.neogeo.org.uk/gps/tracks/20101025_meallfuarmhonaidh.gpx

Just change the lnk= part to point to the address of the file you uploaded.