Ruth, I don't know the exact figure, but the number of horse deaths is much higher than you quote (though some do occur while travelling to/from the fair):

2006
Meanwhile, the RSPCA is investigating the death of a horse, which collapsed and died outside the Grapes public house in The Sands, Appleby, in the early hours of June 12 morning. It is believed the horse was abandoned after being run up and down Battlebarrow, a number of times, before collapsing after becoming exhausted.

On Tuesday (June 6) a horse, which was being ridden by a traveller, was killed after it ran out in front of a Land Rover Discovery driven by a 19-year-old woman from Ingleton at Devil's Bridge, near Kirkby Lonsdale.

2005
Police also believe that a horse killed by a van on the A65 at Newby, near Ingleton, belonged to a traveller.

The horse had been tied to a fence when it was struck by the van, which was written off following the accident, at 4am on Tuesday.

The RSCPA usually records up to five injured horses a day. However, two horses died at the event, one dying from blood poisoning caused by stress. The owners of both horses failed to come forward.

Source: The Westmorland Gazette