MDI's sounds a likely cause. A frequently used ATFM tool to avoid a regulation and the associated NATS attributable delays. Dover sector is a narrow piece of airspace being fed by not just LL deps but GW,SS,KK,LC etc. plus traffic inbound to Brussels TMA dropping through London TMA climbers. MDI's and ADI's are not overly popular with tower controllers as it increases their (already busy) workload and as you experienced, extra time fo a/c sitting on the ground.
But as I wasn't there this is just an educated guess to aid your query