Eurostar is £39, train from Brussels south is E39 (£31), so about £70 in total, so it's also cheap (I believe budget also played a role in the itinerary). And yes, it takes a lot of time, but so does flying. And as there are many more trains, chances of getting disrupted because of the weather are much smaller. I know I shouldn't be talking about trains on an aviation forum, but still...

OP stated he'll be traveling with hand luggage only. In that case BA would be £89, Luxair would be £99. Luxair means much less waiting time (plus that LCY is a very small and efficient airport). Op should then decide if saving a few hours is worth £10.