I think there could well be a sizeable market there. There might be a small seasonal market in the Summer months, but other than that I'm not so sure.
It should be more than summer only. Times have changed. Proportionally the number of Belgians spending money on citytrips beyond the summer period has risen considerably. Many Belgian retirees have moved over to the Belgian coast within the catchment area of Ostend airport. In general they appreciate the conveniences smaller regional airports are offering (less hectic,closer to home,less distance to walk on the airport site itself,shorter check-in times ,less queuing ,etc..). Also nearby ports of zeebrugge and Ghent have grown considerably the last 10 years. On top of that there is no longer a ferry connection between Ostend and UK. Offering air-services from OST would be the fastest connection between Western-Belgium,Northern France and Zeeland(NL) and major UK cities,chunnel or no chunnel. Fact is also that UK(not only London)is getting more attention and fans(maybe due to all those Belgian football players in the Premier league)
It may be true that many routes between OST and UK would not have sufficient point-to-point passengers all year long,but by combining with other city pairs,and with the support of the travel agencies and local tourist authorities , IMHO air connections from/to OST would be a viable extra to the currently offered transportation options.