There has always been a drift to Gatwick. Higher yield of catchment makes it attractive for UK originating traffic. Court Line when they took over Horizon, Monarch when ex Dan Air chief the late Alan Snudden took over. I can remember how disappointed I was in 1983 to learn that Monarch's new 757s would make just one visit a week on a Sunday on a w-pattern form Gatwick.
Luton does have an advantage of lower fuel burn on the routes from Eastern Europe where yield is less sensitive to the airport used, although with Brexit there may be less of this in the future. I would expect these to largely stay at Luton, except where they fill gaps for LGW aircraft.