It was the first time the firm reported a loss from its business in Britain since inception in 2013, the filings showed. However, it did not give a comparative 2015 figure, instead reporting a 8.7 million pound profit in the 18 months to December 2015.
The Moroccan-born French brothers, who previously worked at Morgan Stanley and Goldman Sachs, are among several other experienced bankers to set up small outfits that offer niche expertise and independent advice, taking a different tack to the big banks that tend to sell other services too, like financing.
Since 2013, Zaoui & Co has played a role in major deals, such as the $60 billion (£44.25 billion) merger in 2014 of France's Lafarge and Switzerland's Holcim that created the world's largest cement maker, snatching business from rival advisories and banks.
The filings in Britain are only a partial insight into the business, as Zaoui & Co's parent firm is incorporated in Luxembourg, where results are not published. M&A transactions also take months to complete, so proceeds can be delayed.
The company declined to comment.
Turnover in 2016 was 3.66 million pounds, down from 17.3 million in the 18 months to the end of 2015, mostly coming from Europe, showing the firm's reliance on business outside Britain.
The Brexit vote "could cause disruptions to and create uncertainty surrounding our business, including affecting our relationships with our existing and future counterparties," Yoel Zaoui said in a report signed in April, according to the filing.
Bigger and more established boutiques firms include Rothschild, Lazard, Centerview Partners and Perella Weinberg, which are all also based in London.
Zaoui & Co ranks No. 66 among top advisory firms in Europe so far this year, Thomson Reuters Eikon data show.
It advised on the merger of French and British laundry services firms Elis SA (>> Elis) and Berendsen (>> Berendsen) and the acquisition by Neptune Oil & Gas of a stake in the exploration and production arm of French utility Engie (>> Engie).
It also worked on French carmaker PSA's (>> Peugeot) purchase of Opel and Vauxhall from General Motors (>> General Motors Corporation) and the acquisition by JCDecaux Holding (>> JCDecaux) of a stake in Eurazeo (>> Eurazeo).
(Reporting by Clara Denina; Editing by Edmund Blair)
