Oh, I see. In that case, Dr Otto is not stricktly His Imperial Highness but merely out of courtesy some people still call him as HIH ? I bet he must have done that after the empress died. She wouldn't have liked it if she knew.
Why don't you simply check out:
Otto von Habsburg - Wikipedia, the free encyclopedia -
There you can find beyond many other information this "In 1961, Otto "for purely practical reasons"renounced all claims to the Austrian throne and was eventually allowed to return to his home country in 1966."
The quote is from the book: Gordon Brook-Shepherd
Uncrowned Emperor - The Life and Times of Otto von Habsburg, Hambledon Continuum, London 2003.
ISBN 1852855495, which might interest you.
And here's part of a press release from the Uk which might give you an idea how the Archduke is received officially in the UK - interesting that members of the BRF and some ambassadors from countries which once belonged to Austria-Hungary attended.
11 July 2007
Last Crown Prince of Austria receives the Freedom of the City of London
The last Crown Prince of Austria, Otto von Habsburg was awarded the Freedom of the City of London by the Worshipful Company of Gardeners on Thursday 5 July. The Crown Prince was nominated by Sir Gavyn Arthur, who had just been made Master of the Gardeners earlier that day. TRH Prince & Princess Michael of Kent, Sheriff Richard Regan and the Ambassadors of Hungary, Croatia and Slovenia attended.
Sir Gavyn Arthur said “It’s a great honour to sponsor the prince as a Freeman of the City of London as he has done so much for the City and Europe as a whole”.
Following the ceremony Otto will be made an Honorary Liveryman of the Gardeners an acknowledgement of his support for the company.
Otto von Habsburg is the eldest son of the Emperor Karl making him the last Crown Prince of Austria and Hungary. He has also been president of the International Pan-European Union and a Senior Member of the EU.