I would try to look for a fligh to/from London at e.g. Virgin Express
and then use from London to Vienna Air Berlin/NIKI
It might be a good idea to book "months" advance and use 'regular' airlines: I'd booked my flights about half a year in advance and paid as little
as $ 850* for a round flight Paderborn (no parking fees, just 15 mins from my home), Munich, Chicago, Denver, Albuquerque, Denver, Frankfurt - with Lufthansa and United (* including all and everything, from taxes to service charges etc etc).