It is fine on the Southeastern trains, which are the same rolling stock. We can get the tandem on and off without trouble; on each side of the door there are two rows of only four seats with a wider part of the aisle in the middle, and I lean the tandem against these with the front wheel sticking into the vestibule area.
Their
policy looks promising, since it doesn't mention tandems at all. Southeastern just don't recognise tandems as a separate problem, and are admirably pragmatic.