Give up on this one, methinks.
There is 'no-where' to spot from at LTN
anymore unless you have a car and drive round to the other side of the airport and go up to the crash gate at the side of the runway. Is that still open by the way??
Birmingham isn't a bad idea, good indoor facility, although some of the views have been lost due to building work.
Having said all of this, it's probably cheaper to go abroad with a lo-cost than it is to drive to places like LTN