The 55 runway movements per hour will likely have an average delay factor agreed by all involved and applied by the schedulers. For example at peak times when runway demand is highest, departing aircraft could expect an average delay at the holding point of say 10 or 15 minutes. Same may apply for arrivals depending on where the demand is. It's physically not possible to sustain 55 scheduled movements per hour on a mixed mode runway for several consecutive hours. There has to be a "firebreak" after these peak hours where the scheduled number per hour is reduced so any delays built up during the 55/hour peak can be dissipated. This is common practice in airport scheduling.
Last summer Gatwick scheduled 55/hour for 3 hours of the day only with two consecutive hours at 55 movements but the two consecutive were late afternoon period when there are very few "heavy" aircraft types involved which therefore does not require increased vortex spacing and minimises runway occupancy times which helps towards achieving the 55 movements without much delay, subject to good weather of course. I'd be surprised if the scheduled declared runway capacity for summer 2016 was increased as doing so may increase any delays that last summer's declared capacity schedule created.