In normal conditions, up to 46 flights per hour are scheduled to arrive at London Heathrow. When London Heathrow is affected by bad weather, Air Traffic Control reduces the amount of flights that can arrive at the airport. When Air Traffic Control imposes restrictions on the amount of flights that can arrive, departures are also affected, which results in delays and cancellations.
On the day you travelled, the arrival flow rate was reduced to 26 flights per hour between 05:40z-09:00z, so a number of flights (outbound and inbound on 9 routes) had to be cancelled.