When I rang round the London marinas in early July they were taking bookings several weeks in advance. Perhaps more space now that schools are on their way back, but you definitely need to start contacting them.
Yes you definitely need to reserve.Would suggest a follow up communication shortly before departure to confirm they have not "forgotton" your reservation.Without doubt the best of the London marinas if a tad expensive but a location absolutely second to none.
Alternatives are Limehouse,South Dock and( MoBos Only) Chelsea Marina and Imperial Wharf.All of which are OK but are either in the wrong place or have tiny tidal access periods.
Its a few years ago since I last went to St Kats., but from what several friends have said recently and that which has appeared on another thread, I think referred to, above; I don't think I'll be in a hurry to return. May try Limehouse.........