Founded in London only three years ago by Simon Robey and Simon Warshaw, former high flyers at Morgan Stanley (>> Morgan Stanley) and UBS (>> UBS Group AG) respectively, the firm now ranks fourth in Britain for merger and acquisitions business, Thomson Reuters data shows.

The tiny firm has only 15 employees, yet it has landed advisory roles on some of the largest deals of the decade. Among them were the $100 billion-plus acquisition of British brewer SABMiller by AB InBev (>> AB InBev) and oil major Shell's $70 billion purchase of oil and gas company BG.

Small firms such as Robey Warshaw have been snapping up veteran bankers fleeing bureaucracy and shrinking paychecks at the big players and are proving popular among companies that value their niche expertise and independent advice without the the type of cross-selling sometimes favoured by big banks.

Such boutique firms now earn nearly half of all M&A fees in Europe, winning market share and top dealmakers from global investment banks, Reuters reported in August.

Recently released filings show that Robey Warshaw achieved turnover of 43.4 million pounds in the year to March 31, up from 23.9 million pounds a year earlier.

Profit of 36.6 million pounds, up from 19.4 million pounds in the previous year, was shared by the firm's three partners -- Simon Robey, Simon Warshaw and Philip Apostolides, also a former Morgan Stanley banker.

The highest-paid partner was eligible for 18.2 million pounds of the annual profit, illustrating how lucrative boutique firms can be for senior dealmakers.

Megadeals such as the AB Inbev-SAB Miller and BG-Shell acquisitions drove up global M&A volumes by 41 percent to a record $4.4 trillion in 2015. In 2016, global M&A volumes fell 17 percent from that peak to $3.6 trillion.

British M&A totalled $177.5 billion in 2016, down sharply from a record $394.8 billion in 2015 -- which featured those two megadeals -- but in line with the five-year trend.

(Editing by David Goodman)

By Anjuli Davies

Stocks treated in this article : Morgan Stanley, UBS Group AG, AB InBev