Westwood sank a 40-foot putt for birdie on the 16th hole and a 30-footer for eagle at No. 17 for his second victory of the season. He had a four-round total of 15-under 273.
"I'm delighted to win," said Westwood, who won the Andalucian Open in May. "This is a very prestigious tournament in Britain, and it's only an hour from where I live."
Ian Poulter (70) was second at 278 and was one stroke behind with three holes to play. Mark Foster, the leader after the second and third rounds, shot a 74 and finished third, one behind Poulter.
Poulter said he was deflated after Westwood's eagle putt.
"I had nothing left in the petrol tank, it was empty," Poulter said. "If I had taken my chances today it might have been a different story. I should have brought it home but Lee played great. All credit to him."
The 34-year-old Westwood won 18 times from 1998-2000 and was ranked fourth in the world at one point.
"I'd like to get back into the top five," said Westwood, who was 48th at the start of the British Masters. "I really wanted to stop the rot of being in contention and not winning the last few weeks."
Westwood said an improved short game, achieved with help from friend Mark Roe, has helped him.
"With a good short game to fall back on, it frees up your long game. Whenever I missed a green today, I was able to chip close and tap in," Westwood said. "I was hitting shots I could not have hit two years ago."