Potential acquirers need to move quickly as soon as news of a bid leaks. It's difficult to balance the need for secrecy with the need for bodies on board, but having at least banking advisers lined up and a credible valuation of the business helps.
Going over the board's head, directly to shareholders, known as "going hostile", risks a lot of shareholders' money in a billions of dollars deal, without access to the company's books.
There is also no break fee payable to the bidder if an agreed deal falls through.
One advantage of the speed engendered by the rules is that U.K. companies are also less likely to pursue the "poison pill" defences some U.S. acquisition targets have tried.
There hasn't yet been any evidence that the rules have put off any M&A activity in the interim. The M&A cycle has been static for most of the decade, according to figures from Mergermarket, as companies rebuilt their balance sheets following the credit crisis of the previous decade, and avoided risk.
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In future, some more publicity-averse investors, like sovereign wealth funds or private equity, may be put off, according to Adam.
"There are individual entities who are shy about doing things in the glare of publicity, and this is something certain clients are nervous about," he said.
Greater government say?
There are also concerns that, with the U.K. political storm around Pfizer's bid for AstraZeneca, the government may increase its powers to halt deals.
"At the moment, the opportunity for the U.K. government to intervene is limited. If the Labour Party wins next year's elections, there is a chance this may change, and this may affect the political climate," Bogdanor pointed out.
Pfizer now has six months before it can come back to the table for AstraZeneca – or three if its target's board decides to reengage. There were plenty of reasons other than the Takeover Code which meant the deal didn't work out this time, but the pharma giant will no doubt be even more careful of the rules if it comes back.