It was my understanding the trains in the UK are getting too crowded even for folding bikes! Since they are not going to buy new rail cars, the days of bring in a full size folder like the Dahon Espresso (during rush hour) could be over.
I hardly think anyone is going to take a measuring tape to determine if the 20' inch Dahon exceeds the limit. In fact, most people can't even determine the difference between a Dahon and a Brompton!