What were those multiple flights leaving London night of 29th Aug 2019?
Nothing stands out on Flightradar24 for that time period by using the playback function. So most likely they're not civilian.

What I found was a military-related NOTAM via notaminfo.com/ukmap:


  MIL EXER. MIL PARACHUTING AND HELICOPTER HOIST OPERATIONS WI 5NM
  RADIUS 515900N 0012500E (BAWDSEY, SUFFOLK). FOR INFO 01638 545012.
  2019-08-0649/AS3
  
  FROM: 29 Aug 2019 18:30 GMT (19:30 BST) TO: 30 Aug 2019 00:30 GMT (01:30 BST)


The time matches if we account for padding, and the drop area would make them fly west to east to arrive at Bawdsey, depending on their base and your location.

If too concerned, you can trying calling the number quoted above.