The 1939 reprint of the 1935 Manual of Military Law refers to paragraph 207 of TA Regs:
A man of the Territorial Army may enter the Royal Navy or enlist into the Regular Army, the Militia ( Supplementary Reserve, Categories A and B only), the Royal Marines or the Royal Air Force, and, on final approval, is deemed to be discharged from the Territorial Army, but must return in good order the articles of clothing, &c., issued to him belonging to the Territorial Army. He is not, however, allowed to join the Royal Naval Reserve or the Army Reserve ( other than the Supplementary Reserve, Categories A or B.)"
Paragraph 204 was among a number of paragraphs relating to varying reasons for discharge. As you say your man was joining the regular army it would seem to suggest he was doing it from the Territorials.
I acknowledge the assistance of an expert from another place.
MaxD