When you visit Brussels London is probably not the first city you think of visiting while in the capital of Belgium. But, since the arrival of the Eurostar trains between Brussels and London this is like a piece of cake to do. If you travel with one of the Eurostar trains from Brussels Midi to London you can reach your final goal in two hours, and the prices are not especially high, at least not if you book some weeks before your travel date.
London does not need any further introduction, but in short terms it is one of the most important cities of the world and some hours or one day is not at all enough to get to see this city. But, on a day trip you can still get to see the most important London attractions, maybe go for a trip with the London Eye and eat some fish&chips.