I knew there was a catch, which can be found on page 105:

I think I have worked out from further reading that this means that at some point between 2010 and 2015 we will have a reduced peak service to London Bridge, but longer trains. Assuming the plan may be to reduce the services from 6tph to 4tph in the peak but increase from 8 to 12 carriages, we get the same capacity but must wait longer for a train (hence 'unpopular').

This is still only an option for consideration and would require work to be carried out to extend all the platforms on the route before 2012 or whenever the construction work would take place. But if there is no loss in capacity it may be a reasonable sacrifice to get longer trains and a direct connection to Kings Cross at the earliest opportunity.