Why did this aircraft do a ‘spiral’ before landing?
The flight was on the 28th, and before orbiting it was around 12:00 UTC. By using the playback function of FR24, this is the view:



The flight in question was 2383 with call-sign WZZ1161 (FlightAware permalink).

The boxes with numbers and arrows indicate the flight level and either climbing or descending. As you can see, two busy streams were approaching the airport from the north. This orbiting as opposed to a holding track (race track shaped) is common and its phraseology can be:


  for spacing orbit left
  for spacing make a 360 turn left


Fast forward:



And a Ryanair flight did the same before continuing the approach. It was just heavy traffic and it's quite common over Heathrow (live as of writing this):