* Biggest U.S. cable operator swoops for Sky
* CEO Roberts learned "a lot" from London taxi driver
* Roberts had "keen interest" in Sky for many years
LONDON, Feb 27 (Reuters) - When Comcast boss Brian Roberts was weighing up a $31 billion offer for British broadcaster Sky, a trip to see Sky's products in store, and the knowledge of the London cab driver that took him there, helped to make up his mind.
Chief Executive Roberts said the visit was one of a number of things that confirmed Sky as a "jewel" that his company should try to acquire, as Comcast looks to boost international revenues as growth in the United States slows.
The biggest cable operator in the United States offered on Tuesday to pay $31 billion to buy Sky, challenging Rupert Murdoch's Fox and Bob Iger's Walt Disney for the European pay-TV group.
Roberts said he had closely tracked what Sky had been doing for years, but last November, he had an unexpected reminder of Sky's influence as European's biggest pay-TV provider while in a cab with a colleague.
"I suggested that we jump in a taxi and go to one of the malls and get a demo of Sky in one of their shops. And we had a fabulous experience," Roberts told reporters on a call.
"The cab driver was incredibly knowledgeable about the difference between Virgin and Sky in every feature. We were learning a lot there. Then when we to the Sky store, we spent at least an hour going through every feature and comparing it to our own... We were really terribly impressed."
Liberty Global-owned Virgin Media is a British rival to Sky.
Roberts said that while the experience wasn't the deciding factor in moving forward with the bid, it reiterated the media group's value to the chief executive.
"It was another reminder for me how impressive Sky is, and how lucky we would be at Comcast to be able to combine together."
Another element in Comcast's thinking was Sky's success earlier this month in winning the rights to show Premier League soccer matches for a fee well below analyst expectations, boosting its future earnings and lifting its shares.
The offer is a huge statement of intent from the mild-mannered media mogul Roberts, 58, who took over as head of the cable and entertainment conglomerate from his father, Ralph, who founded the company in 1963.
The younger Roberts became Comcast's president in 1990 and CEO in 2002, having spent his whole career at the company.
He told reporters that he had taken a "keen interest" in Sky for many years. He said Sky's chief executive Jeremy Darroch had shown him an early preview of the Sky Q TV platform box, and that several people had worked as executives for both Comcast and Sky.
While Fox's 39 percent stake in Sky and the agreed takeover by Murdoch has produced complications around the company, it also presented the opportunity to bid, and Roberts said that his London visit was a reminder of Sky's fundamental value.
"Frankly there's nothing as great in the United States," he said.
"Seeing it again, and listening to the passion of the sales and looking at the product... all those things combined to reinforce what a number of us have known for years - that this is a jewel."
(Reporting by Alistair Smout; Additional reporting by Eric Auchard and Kate Holton; Editing by Georgina Prodhan)