Educating the Normandie public was very effectively done by CFR who had the advantage of having the full resources of the Chamber of Trade & Commerce who also hold the management contract for the airport. The cost of doing so was still substantial however.
I recall that for one of the early months of the SEN-CFR-SEN flights the mix was 40% UK and 60% French pax (taken from passport details). That obviously doesn't automatically mean that exactly 60% of traffic was inbound to UK but I think it gives a fair indication.