Yes, suggest London first as follows on the transAtlantic:

13Dec - DTWLGW
23Dec - BRU(AMS)DTW
cost $709.59

If you were to fly into LGW and return from AMS (train to BRU and back), cost is $607.93.

Adria Air has flights LGWLJU ($108) and LJUBRU ($421), but if you fly LJUAMS instead ($133). You'll have to play with dates as some of these flights/fares are not available on the dates you have listed.

Have a good trip...