This is just an observation but I think it is accurate. In 1999 most of Luton's passengers were UK citizens as it was rare to hear a foreign language inside the terminal so I will call that 5 million. I would say that figure now sits at around 8 million or 9 million so demand from UK citizens has grown say 80% in nearly 20 years.
Passengers from outside the UK has grown from next to nothing to 8 or 9 million so that is where most of the growth has come from. This is mainly due to Eastern Europeans and Luton being more recognised as a London Airport.