Hallo Wolfgang,
I have searched the British archives to see if there is still a record of
the naturalisation/denizenation of your ancestor Johann Buchorn/John
Buckhorn.
http://www.nationalarchives.gov.uk/catalogue/default.asp?j=1
On the English census, my gt.grandfather Hermann Flemme/Flamme also wrote he
was a British Subject (BS), but there is no record in the British archives
of his application.
Below is the wikipedia explanation of why an immigrant would want to be
naturalised to become a citizen of England. (Maybe you can find the German
website translation). It was only when immigrants wanted to vote or to own
property that they requested naturalisation. As most people rented their
homes and shop premises there would be no need to apply. Also, as your
ancestor and mine both had English born wives I would question whether they
had to apply for a piece of paper. Furthermore, because the Kingdom of
Hannover was a British protectorate I believe English Common Law would apply
to Hannoverians.
I have looked on this popular website to see if anyone is researching
Buckhorn/Buchhorn - unfortunately not, but possibly if you ask on the
"Immigration Board" you might find some help.
Is this your entry on genesreunited?
Regards,
Rena in England