Here is some information I picked up.
"Starting in July 1941 the RAF started to receive modified P-39 that the British called Airacobra Mk. I. The original order was for 475 aircraft under the Lend-Lease Act and they would also take over the 200 aircraft France had ordered before it fell to the Germans. Because of the planes inability to climb to high altitudes and its relatively short range it was deemed inadequate. As a result only 80 of the planes were actually accepted by the RAF 601 Squadron. No. 601 Squadron RAF was the only British unit to use the Airacobra operationally, receiving their first two examples on 6 August 1941. On 9 October, four Airacobras attacked enemy barges near Dunkirk, in the type's only operational action with the RAF. The squadron continued to train with the Airacobra during the winter, but in March 1942, it re-equipped with Spitfires."
