It would make sense for Leaky Cauldron clients (i.e. those paying to eat at the restaurant) to be able to access Diagon Alley in a more authentic manner after they finish dining. After all, it's easier to control a dining queue than a park entrance queue. Maybe that "hidden" Leaky Cauldron door next to King's Cross has something to do with that?
In the concept rendering below, however, the hidden Leaky Cauldron door is completely ignored. It almost looks like there is a separate queue leading from King's Cross to the area beneath the Leicester Square Station marquee, and a larger entrance area next to it, also under the Leicester Square arches but not the marquee. There are several doors and explosive light(!) coming out or leading to Wyndham's, but I can't tell if the people are going in or out. This may be older concept art, however:
In the other concept rendering I've seen of the waterfront, that area is completely blocked by tree foliage, of course.