Peter,
That is the one however to get a match to the level of detail of the Farish coaches go for one with the plainer detailing these are quite a bit cheaper.

Yes re length. the BG is 57 foot and M3s are 75 foot  in length. As a result the bG is a go anywhere vehicle and the MK3 when built was restricted however a lot of improvements made for 156/158 sprinters and the like making the Mk3 coach route availability a lot better. 

If you can get a BG with B4 bogies rather than a B1 then a lot more true to reality, as BG would then 100 or 110 mph max matching MK3 on say a West Coast train.

festive greetings
Robert