I have a bottle of House of Commons blended whisky signed by Tony Blair. I believe he signs bottles and other souvenirs (e.g. the label of a cuddly bear, creating a Tony Bear) and offers them to Labour MPs for raffle prizes or for fundraising auctions. Some of the Ministers use them as prizes for their Departmental Christmas parties. I suspect they are not that rare and they could be forged easily but they are highly valued by the people who win them and believe in their provenance.