If not a bullaun, I'd say it's very unlikely to be a preaching cross socket. At 10cm deep, 10cm wide hole isn't going to support much of a cross with a relatively small mass of stone around the hole.
There's such preaching cross socket at the holystone
http://www.themodernantiquarian.com/browse.php?site_id=1791#post-17363
and though Iv'e never found it, Iv'e had it described by the owner as rectangular with a hole right the way through. If that's what preaching cross sockets (What an unwieldy term) look like then it must rule out both the NYM one and the Copt howe one as preachingcrossockets. So what are they? Mortar and pestle would surely be the bookies favourite in the absence of any other theories.
Pre-emptive derisory snorts to any suggestion of plinths for stone crosses.