MADRID/HONG KONG (Reuters) – Li Ka-shing’s Hutchison Whampoa Ltd has agreed to buy Telefonica’s British mobile unit O2 for up to 10.25 billion pounds ($15.4 billion), as Asia’s richest man makes his boldest bet yet to revamp his European telecoms business. Hutchison already operates the Three Mobile network in Britain, and buying second-ranked O2 from the Spanish group in Li’s biggest ever takeover will make it the top mobile operator in the country.