Heathrow, like most airports has to submit to a national aircraft slot agency. From time to time it reviews the availability of slots and reallocation occurs where no agreement to buy or sell exists between airlines. Strictly speaking that process isn’t actually legal but it’s tolerated.
This normally occurs where airlines have failed and – such as JetAirways – simply abandoned their slot, or given them up for one reason or another.
Heathrow’s slot distribution is also governed by compulsory U.K. public service slots that can only be used for domestic short haul.
As a result of a new review, where low and medium cost long haul airlines are seen as under represented at Heathrow (something of an understatement), Norwegian was offered, and has accepted 3 slot pairs – giving it three departure and arrivals per day.
Norwegian hasn’t decided what to do with them yet but Heathrow is pretty much the Holy Grail of European airports, with capacity at 98.9% its rare enough to ever get access to newcomers.
A mix of long haul and European feeder is quite likely. With Copenhagen and Stockholm loosing long haul flights London would make an ideal transfer point. Once Norwegian can be seen to make the slots work it’s in a position to take up more later.
British Airways are going to hate this so much, and Virgin Atlantic won’t be too pleased either. Between them they’ve managed to keep a lid on low cost long haul at their primary airport.