Sounds like you prefer to do BRU-ORD-DCA (cheaper, later departure) over BRU-LHR-IAD (more expensive).
The Star Alliance lounge at Heathrow T1 is a bit nicer than a United Club inasmuch as there is a little bit of hot food, and an open bar, and showers.
On BRU-ORD-DCA you'll clear customs / re-check bags at ORD. On BRU-LHR-IAD you'll stay in transit at LHR and won't have to do anything special re: immigration.
Right now SN and UA both use LHR T1 for BRU-LHR / LHR-IAD.