PAXboy - it is a little unfair to compare Johannesburg with Luton. One is the primary airport for not just a country of 49 million people, but in practice an entire region of Africa - flights from Europe to places like Botswana are not common. As such it will tend to attract the premium traffic, and thus not only will its airline customers be prepared to pay more, but so will the passengers.
Luton on the other hand has Heathrow and Gatwick way ahead in the pecking order, with City picking up the high-end business traffic. For many years, Luton lived off holiday charters to Spain. Yes, there is a bit of private aviation but this tends to have minimal involvement with the main terminal. The people for whom the terminal was built are the great unwashed who are generally looking for value for money rather than a plush experience. This means that revenue per passenger will be lower, and Luton has to manage its affairs commensurately.
Building bridges between areas of a terminal and installing + maintaining lots of travelators doesn't come cheap !