Michael O'Neill, who recently resigned for health reasons as CEO of Britain's Barclays Bank on his first day in the office, is far from typical of Americans chosen to head up big British groups. Most stay longer. European companies often prefer to hire an American in order to avoid cultural clashes with their new merger partner. Reed Elsevier, the Anglo-Dutch publishing group, is grappling with this problem, and a U.S. leader is generally regarded as the likely solution. Reckitt & Coleman, a British household products maker, also is looking across the Atlantic. It hopes an American could help it head off takeover threats. British headhunters also like recommending Americans for the top spots. Big pay packages for the incoming executives mean beefier commissions for the people who find them.