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Normally, on a 6-hour layover, I would leave the airport and explore the city, but the immigration lines at Brussels Airport can be absolutely brutal and are best avoided.
Since you’re connecting to the UK, you shouldn’t need to enter the Schengen zone, and it’s definitely best to avoid it if possible.
To determine which airline you’ll be flying with, you can check FlightAware for the tail number of the plane they have been using on this route.
Premium Economy on Lufthansa Group airlines (which includes Brussels Airlines) is usually perfectly fine, though nothing extravagant.