Well, Stanier was Swindon-trained, but he did his best work for the LMS and for them produced the 8F and Black Five, surely two of the finest and most versatile British steam locomotive designs ever. And his deputy was Bob Riddles, an underrated engineer whose talents British Rail didn't use to their best advantage. So I claim my point for that one!
Gresley? Well, point taken about him being best remembered for his express designs, but his mixed traffic locos weren't disasters either - especially, as you say, the V2. Granted, he didn't produce a heavy-freight design, but then he didn't need to: the LNER was awash with ex-War Department Robinson 2-8-0s. Nor did the GWR produce one in the Grouping period, since G.J. Churchward had done the hard work for them before WW1! Lest we forget, in the last years of the LNER, Arthur Peppercorn was a damn good engineer too, and Thompson deserves an honorary mention.
Bulleid? Well, point taken about his eccentricity, but many of his designs did excellent service once the weirder bits were taken off (although granted, BR did that)! And anyway, I think the Southern deserves 'innovation points' for pushing through electrification in the face of a difficult economic situation.
The LMS and LNER both dabbled in mainline diesels and the former had an active programme to replace steam locos with diesels for shunting: the GWR, contrary as always, carried on building steam shunters right up to nationalisation (and beyond)! I won't knock the GWR's engines at all - there's nothing like the sight of a Castle or a King in full flight, and they were sturdy and reliable too - but they were looking old-fashioned by 1939...
<stands on top of an LNER B1, making rude gestures in the direction of Swindon>