Yes, but Ryanair tried Ostend from London. The SE is well served by the Channel Tunnel and ferry connections; Ostend is actually not far from Calais. It is not surprising that an Ostend service did not succeed in the SE market.
Manchester and points north are a different matter entirely. Ferries from Dover to Calais and/or the Channel Tunnel must be accessed via expensive and time-consuming overland journeys. The London area must be transited. Alternatively, long North Sea ferry crossings could be considered. But whichever way you tackle it, Oostende/Brugge is not an easy journey from Northern England and Scotland.
Mr A Tis has it right. The region surrounding Oostende/Brugge is attractive for tourism, there are no airport constraints and flying time from MAN should be just about an hour. The fact that RYR failed in the saturated London market should not exclude consideration of a market with quite different dynamics.
With regard to overland travel from Charleroi to Ostend, this can be done but it is not an ideal choice. Probably 3 to 4 hours by train. There used to be a through service but I'm not sure whether there still is; you'd probably require a change at Brussels. Ostend from Brussels Zaventem is around 2.5 hours by rail, usually with a change in Brussels.