I quite like it, but think the narrowness of the rooms would give it quite a cramped feel, and noise from neighbours each side could be quite invasive in all the rooms, depending on insulation (or lack of) on the party walls. The price seems crazy, but that's London for you.
The garden annexe looks like it was done to quite a high spec, and could suit a couple who want to move an elderly relative in there, or someone who has a grown up child who can't live independently for some reason, but could enjoy some level of privacy there, while having parental support nearby.