Cheapest flight to London is with Delta via the US. It cost around 1200USD, however the connection/stopover is of around 7 hours, also if she doesnt have a British passport she will need a visa for entry to the US which is another 100USD. I looked at all the options for this particular flight as I'm travelling to London with my wife and daughter in July, in the end I plumped for Air France, 1900USD as the trip via the US is just too long with a toddler.