After 7pm you can take a bike on all of the Metropolitan.
Yeah, ok after 19.00
But it's a pain in the butt on the Met Line stock.
You can't really leave the bike anywhere without it blocking either the doors (which open on different sides every few stations) or blocking the aisles/seats.
The only place is one of the carriages with a single door. But you'll have to sit on one of the little pull down seats so you can hold onto the bike. It
will fall over if you leave it there alone.
Getting in and out of that single door isn't easy with a bike and sitting on those little seats for 70 minutes will be a
real pain in the butt.
Marylebone all day long.
< edit >
Chiltern Cycle policy is ok to travel after 19:30.
Trains to Amersham (Aylesbury) are 19:27
19:57, 20:27
Takes 35 mins