The FTSE 250 has been the U.K. stock gauge to watch if you want a sense of how the country’s economy is handling the Brexit process.

But it’s now in for some competition, as the CBOE Holdings Inc.’s CBOE, -1.78% Bats Europe exchange and data provider FactSet FDS, -0.32% have teamed up to launch the Bats Brexit High 50.

This new benchmark is made up of the 50 companies in the Bats U.K. 100 Index (a year-old rival to the FTSE 100) that get the lion’s share of their revenues from the U.K.

Right now, the Bats Brexit High 50’s components are underperforming, as shown in the chart above. The gauge is roughly flat (up only 0.8%) since Britons voted on June 23 to leave the European Union, according to Bats.

Meanwhile, the Bats UK 100 Index is up 16%, and the FTSE 100 UKX, -0.52% also has gained roughly 16%, both helped by the pound’s GBPUSD, -0.03% 17% slide. Sterling’s slump has been a boon for these two indexes’ multinational components, as they generate the bulk of their revenue overseas and then translate the money back into pounds.

Another new index that is all about non-British revenue is faring even better. The Bats Brexit Low 50 — made up of the 50 names in the Bats UK 100 Index that get the smallest portions of their revenues from the U.K. — is up 23% since the referendum.

So the domestically focused Bats Brexit High 50’s underperformance could inspire “Remain” supporters — those who wanted to stay in the EU — to say they were right in their warnings about a withdrawal.

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It is telling a somewhat different story than the FTSE 250 MCX, -0.53% , which is up 9.5% since June 23 and was notching fresh record closes earlier this month.

That mid-cap index’s gains show that Brexit fears were unfounded, said Jasper Lawler, senior market analyst at London Capital Group, in a note last month. Lawler also said there are still many big companies with large foreign operations in the FTSE 250, so it’s not the best gauge of domestic sentiment.

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