Loading a supplement onto a Luton flight rather than the equivalent at Gatwick probably means the passenger will incur additional costs to travel to/from Gatwick thus negating the value saved on the flight.
After all, Gatwick isn't the most convenient airport for everyone I the South East! A more flexible approach that delivers a broader range of flights, destinations & flight times from airports both North & South of London might deliver better options for passengers.
Perhaps I'm being too simplistic but 3 aircraft in summer & 1 in winter seems a very paltry amount when seen in comparison to the vast options out of Gatwick. Luton doesn't even compare with airports like East Midlands when it comes to Thomson. It would be nice to see a couple more planes & a better range of routes from Luton but I won't hold my breath