Hostile M&A is making a comeback. But there is one problem for foreign bidders looking to do deals in the UK: the rules of engagement have changed.

After Pfizer's failed final offer for pharmaceuticals rival AstraZeneca last week, bankers and advisers were quick to point out the difficulties faced by bidders trying aggressively to take control of reluctant UK takeover targets. It is a difficulty that stems from the UK's Takeover Code, which was enhanced in 2011 and offers a strong negotiating position to the target company.