In September 1809, the Brunswick corps was organised into one infantry regiment of two battalions, 12 companies in total, and one hussar regiment of 6 troops (not enough men to form 10 troops, as originally intended to maintain the original structure in Germany).
When the corps was formally taken over into the British army in November 1809 (dated back to 25 September 1809), this structure was maintained (see p. 416 ff.), but the infantry was formed as one single battalion only. The Etat (p. 419) includes two majors, and provisionally ("vor der Hand"), these were allowed by the regimental commander to command each of them half of the infantry as a battalion (p. 427). Unfortunately, Wachholtz ends his recollections with the arrival of the infantry in Spain, so he doesn't talk of the organisation there.
On p. 405, Wachholtz mentions some disciplinary problems with "die grüne Jägercompagnie" (the green riflemen company), the wording makes clear that there was only one. Accordingly, pp. 420, 432.