Westmoreland began racing motocross in 1995, and switched to circuit racing in 2001. He was runner-up in the British 125cc championship in both 2005 and 2006.
In 2007 Westmoreland moved up to the British Supersport Championship with the Centurion Racing team. Entered in the Cup class for private teams, he came 9th overall in the standings with a best of 4th at Thruxton, winning the privateers' cup that year. In 2008, he finished 4th overall, with second places at Knockhill and Croft.
In 2009, he set up his own JW Racing team just 2 weeks before the season, to campaign Glen Richards' 2008 championship winning Triumph Daytona 675. He started the season slowly with midpoint finishes, but he came into form at Snetterton, grabbing a 3rd place; results continued to improve with a 2nd at Knockhill, and his first supersport win came at Brands Hatch GP. A second win at Silverstone sealed third place in the championship.
For the 2011 season Westmoreland moved up to the British Superbike Championship with the Motorpoint Yamaha team, his first season riding a full superbike. However he failed to produce the results that were expected, and after 8 rounds he was released from his contract, to be replaced by Tommy Bridewell for the remainder of the season.
He rode a Kawasaki ZX-10R for most of the 2015 season with the JG Speedfit Kawasaki team until unexpectedly losing his place during the BSB meeting at Assen in September.
In 2016, he competed in the British Supersport Championship aboard a Yamaha YZF-R6.
