Easyjet have been saying for almost a decade that they are interested in opening a base in Heathrow if a third runway is built. The previous easyJet CEO reckoned on a double digit size fleet based there. A BFS LHR route would be quite likely I would have thought considering the total size of the Belfast-London market. LHR is one of the last few major airports in Europe without an easyJet presence.
2018 BBC article.