The Crosby Garrett Helmet, named after the village near where it was found, is to feature as part of a Royal Academy show called Bronze, the BBC News reported.
A private collector outbid a Carlisle museum at the auction in 2010.
Bronze co-curator David Ekserdjian said the helmet, which is almost 2,000 years old, was a "masterpiece".
"Even if this helmet had been known about for as long as anybody can remember, we'd still want it in the show because it's a fantastic work of art," he said.
"But the fact that it came out of the ground in 2010 doesn't fail to add to its interest and appeal. This is a masterpiece that we didn't know about until recently and that's absolutely thrilling. That combination of quality and novelty is a win-win," he was quoted as saying by the BBC.
It would have been worn for cavalry sports rather than battle it is believed.
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The helmet is believed to be one of only three of its kind found in Britain.
The Ribchester helmet found in 1796 is housed in the British Museum and The Newstead helmet found in 1905 is in the Museum of Antiquities, Edinburgh.
Other highlights at Bronze include the severed head of King Seuthes III, excavated in Bulgaria eight years ago and never seen in Britain before, and the 14th Century BC bronze and gold Chariot Of The Sun from Denmark.
Bronze runs at the Royal Academy Of Arts from September 15
To December 9.