BRU isn't that well organised, unfortunately. All my ex-UK arrivals (which are sterile flights) have been landed into the general arrivals zone, meaning you have to go through transfer security again before going back upstairs. (It's ridiculous, but BRU have never bothered to reply to my intermittent questions about why they can't land UK flights into the sterile departures area).
You need to be at the gate earlier for secondary screening and passport revalidation. You may get randomly selected for an additional pat down and bag check.