Looking through some of Rene Chartrand's Osprey titles, specifically the two on émigré and foreign troops in the British Service (1793-1802 and 1803-1815) there is some very interesting information on the origin and service of very obscure units of all arms (infantry, cavalry, artillery, and engineers). Before continuing, it should be noted that Rene Chartrand stands out above the great majority of Osprey authors and is scholarly in his writing and approach and meticulous in his research.
Anyways, back on topic.
Most interesting for this discussion is the French émigré Engineer Officer Cadres which was assembled in England in September 1793 as well as in Ostend in December of that year. They were taken into British pay on the Isle of Wight in early 1794 and about 15 of them served in Flanders with the British. A 16-officer 'brigade' was formed in April 1795 and served in the Quiberon expedition, Portugal in 1797-1802 and at Majorca, the West Indies, and Egypt. This is probably, or most likely, the unit that La Picard de Philippeaux belonged to when he showed up at Acre with Sidney Smith. More research needs to be done on that idea, but it does make some historical sense. Philippeaux did, however, join the British army and can be considered a 'British' officer.