Platform 20 at Waterloo is used quite often. The other Eurostar platforms have seen use during the works which closed platforms 1-6 last summer, and also for diverted services from South Eastern during the last stages of the Thameslink project at London Bridge.
They are supposed to be being readied for the new more frequent timetable in December, except that isn't going to happen now because no-one checked whether the electricity supply was up to the job of powering the extra trains.