How low cost do you want low cost to be, and how annoying are you prepared for it to be? I just picked (fairly arbitrarily) Monday 19th August, and although Expedia does offer a GBP18 Ryanair flight SXF - BRU you have to accept an 0620 departure time to get that, and of course no checked baggage and you need to include costs of getting to/from the airport. Given the need to arrive at least 1h before departure I would personally not even consider 0620 dep, and all the other flights Expedia suggests cost 90 quid or more. For the train loco2.com has one-way tickets Berlin Hbf -> Bruxelles-Midi from GBP63.50 at reasonable times.