There are zero airlines offering flights to London. The most direct route isn’t necessarily the cheapest – being flexible is key to finding the lowest airfare rates. For example, try searching by month to see the best time of the week to fly.
London Luton is just one of the airports in London. If you're not set on a specific airport, try changing your search from London Luton to London to see a broader range of flights – they might be cheaper.
If you're planning on going to London during this time, you could take a look at other airport options. Or you could try to get a last-minute bargain.