A Dahon (I've got one) is undeniably bigger folded than a Brompton. However, it's not that much bigger, even though it does push a strict reading of the limits that you mentioned. A Brompton gives a decent ride and a fold that's pretty-much unbeatable. Once you're prepared to compromise on folded size and go to 20", I think the Dahon family are similarly hard to beat, and I like the ride better. Obviously a 20" Brompton would have a better ride too, but it would be entering a crowded market and giving up its USP to boot.
In practice, almost anything seems to go on our line. Whilst the vast majority use Bromptons, and what seems like a significant and increasing minority Dahons, we get the odd 24" folder and other very large packages, and even the occasional road bike stacked with its wheels out. Also one or two Airnimals, which fold smaller than many 24" bikes, but substantially larger than the limit - a clubmate uses one when getting the train into town and riding the 40-odd miles home.
I've never seen anyone questioned, even when one or two took unfolded bikes - although those simply don't fit on the more crowded trains. I've had one or two trains I couldn't get on with a folded Dahon.
