On 4 November 1942, Thoma was captured by the British forces and for the remainder of the war he was a prisoner in several senior officer prisoner-of-war camps in Great Britain, including Trent Park, Wilton Park, Grizedale Hall and Island Farm. Thoma was subject to surveillance by the Secret Intelligence Service and while speaking to another POW, General Ludwig Crüwell, was recorded discussing rockets that were being tested at Kummersdorf West, which he observed while on a visit that also included Field Marshal Walther von Brauchitsch, the Commander-in-Chief of the Army and other technical programme details.