Luton to Heathrow- how much time needed?

National Express coaches leave at 12:40, 13:40, etc and take 1hr 10 minutes

If she is very lucky she will catch the 12:40 and be at Heathrow at 13:45 which would be just enough time. If she misses the 12:40 she won't get to Heathrow until 14:50. She will than have to find the correct terminal and check in desk.

Train will take about 1hr 35 minutes and are much more frequent, about every 15 minutes. Shuttle bus to Luton Parkway then train to either St Pancras, Underground to Paddington, Heathrow Express to airport or

Luton Parkway to Kings Cross ThamesLink, Piccadilly Line to Heathrow.

The train to St Pancras is the only option which would almost guarantee she arrives in time to make the connection. It is also the most expensive.

If she lands in Luton at noon she'll have to wait for her bags so It'll probably be 1pm before she's on a bus or train. There are several ways to transfer between airports but they're not exactly cheap and can take up to two hours depending on availability, traffic and other factors.

Flying to the US involves checking in a MINIMUM of two hours before departure and there are always lengthy queues to check in and long lines to go through security and immigration. If I was her I'd leave at least 6 hours between flights - just to be on the safe side.

Take the train into London Kings Cross station and then take the tube to heathrow.

Marykin is right - the journey time between airports is likely to be in the region of three hours.