tycho wrote:
The only thing the government can do is to refuse an export licence, temporarily, to allow interested parties in the UK to try to raise the money to exceed the sale price. The ideal solution would be for a multi-millionaire (who can we petition?) to buy it and present or loan it to the the nation for public display in a suitable museum (NMM, Royal Naval Museum etc.)
However would the present government grant one. After all, its only a bit of old jewelry that belonged to Britain's most well known hero, and it's just history and water-under-the-bridge now - like the holocaust, which equally hasn't any relevance today and which has apparently been removed from the history syllabus in schools!
Sorry, perhaps I'm being a little unfair, I don't now what came over me!

Perhaps I should take my own advice and go and lie down in a darkened room for half an hour!
Seriously though, which department is actually responsible for granting such licences? I would imagine that any of the museums mentioned would have a great deal of difficulty in raising the asking price, even collectively, so hopefully as Anna suggested some patriotic multi-millonaire will come forward (or maybe as you also suggest, we should try and think of someone to approach.) It really would be tragic if this important element of Nelson's decorations were to go abroad. If it, hopefully, remains in the UK, I think the best place for it would be at the NMM, on display next to Nelson's uniform coat (suitably protected of course!).