I am no expert on these early tractors, but here is what I've been told & read in books.
The IH Farmall M was one of the most popular tractors imported here during WW2, as wide front for mixed farming use, and single front or V twin fronts for use on flat vegetable growing regions.
They also imported the W6 Standard = both models were only available with vaporising oil engines which started on gas. They did NOT import any wheeled tractors with the gas start diesel engines - but they did import the TD6 crawlers with those diesel engines
After WW2 ended IH GB continued to import the wide front Farmall M until 1949. Then IH GB built the M in Britain using IH GB made parts & castings. The first models were released in 1952 {?} with the IH GB designed engines.
The vaporising oil engine {fuel called TVO = tractor vaporising oil} & the indirect injection start diesel engines.
By the mid-1950's TVO engine tractors were being phased out due to the cheaper diesel fuel. IH GB continued to make both types of engines, but most TVO versions were exported to other Countries.
Note the IH GB re-designed front axle mounting.
The IH GB tractors were badged as BM with TVO engine & BMD with diesel engine