Post spy outside English royal house (11)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

PLANT (post), AGENT (spy) containing [outside] E (English). The line of English kings from Henry II (1154) to the death of Richard III (1485). Now that I’ve come to write the blog I can’t think of an example where ‘post’ and ‘plant’ are interchangeable. Can anyone help me out please?