From what I understand, flights inbound to the UK are operating normally. The return flights are operating sans passengers (I assume belly freight is still flown).. From a legal point, my take is that airlines cannot simply cancel the whole flight if only one leg is subject to restrictions without having to pay EU compensation and possibly damages to passengers stranded on the continent. So I guess those with lots of passengers connecting from long-haul flights (and thus faced with six-digit compensation claims) are more inclined to operate than others.