you could do that mileage on a brompton (especially with Kojaks tyres fitted)... I used to do that sort of miles 8miles each way ( going from my office to the Olympic park three times a week
ontop of commuting via bike and train at start and end of each day.
Flat bar brompton is best bet.
(waits for howls of anti brompton people)
but if you want a 'faster' bike then look at Airnimal Joey or Chameleon
the fold is not so good (you need to take front wheel out) and the stem comes apart rather than folds (but you can twist it through 90degs instead.
They ride really nice though, like a nice road bike, though tyre sizes is weird and limits choice.
Other option would be a Swift Folder
It can be bought in steel frame only from the states and takes std dimensioned components so you can use mtb/road hubs etc, can take derailluer, SS or hub gear.. also uses very common 20" 406 wheels so HUGE range of tyres from fat to ultra thin
the fold is not compact, but it is a stiff and quick and mega flexible build up options.
You can get a aluminium version in the UK and is badged as the Xootr Swift
I don't think much of bike friday, the fold is poor (chain comes off) and they are very expensive.
Moulton is very nice... and as it looks to train people 'like a folder' then you might be ok to get it on a train if it is not too busy without separating it..
separating is not to hard as it has cable quick connectors to separate the rear brake and gear cables, but def not a folder.