For various reasons - partly lower taxes but mainly market forces - it can be considerably cheaper to start a long-haul itinerary in another European country eg Ireland, Belgium or Norway, connecting in London on to the same flight you would have taken anyway.
There is extra time & hassle involved - you need to buy a positioning flight to get to, say, Dublin in order to fly back to London to catch your long haul flight, but for many the savings are substantial enough to make it worth it. A business class ticket from LHR to JFK, for example, might normally cost around £2k but if you start in Dublin that can fall to around £1k.
