No the A340 cannot operate long haul passengers flights from Luton's runway because it's too short. The plans are is that the A340 would fly to Birmingham and Manchester direct. The Luton route would stop in Istanbul and will be operated by an A320, which can be easily operated from Luton's runway.
Seems a bit silly I think. Would make much more sense to fly the A340 into Stansted or Gatwick and if not possible to fly them all on A340's due to lack of A340 aircraft top operate them all or whatever, then surely the Birmingham connection should be the A320, not London!