Frankly I am unconvinced the topic of who did and did not wear a cuirass in the British or French cavalry service during the Seven Years' War has been adequately researched. We've had a collective hive mind view they did not going back to the late 1960s. This has been based on very little first person evidence or citations.
I've had Cecil C. P. Lawsons, A History Of The Uniforms Of The British Army for forty years. When rereading some of it in the spring of 2009, I found this in Volume II on p. 145.
British Heavy Cavalry Cuirass
“5th July 1758. The Royal Regiment of Horse Guards being ordered on immediate Foreign Service and it being necessary they should immediately be provided with a complete set of cuirasses and skull caps, which they have not time to provide in the ordinary way, I desire you will acquaint the Board of Ordinance that it will greatly facilitate the service if they will give directions for them being furnished with complete sets of cuirasses and skull caps from H M stores, on their being replaced by others or paid for by the Royal regiment of Horse Guards as the Board of Ordinance shall think fit.” Barrington
“In 1760 the 3rd. Horse (6th Dragoon Guards) and 4th Horse (7th Dragoon Guards) were also provided with cuirasses and skulls.”
Today (December 29, 2014) while readingBrent Nosworthy's, The Anatomy of Victory Battle Tactics 1689-1763 this appeared on p 132. “Initially, the British cavalry had no body armor, but in 1707 Marlborough gave his cavalry a cuirass in front.” Yes this is fifty years before the Seven Years' War. It is still an eye-opener.