Is a 3h 30m layover between LHR and LTN enough?

On two tickets - no way. If you suffer any delays at all, you'll miss the checkin deadline for the onward flight, and you'll have to buy a brand new ticket. It's just too tight

On one ticket, if you miss the second flight you'll be rebooked for free, so it just depends on how long you'd have to wait for the next flight and how much of an issue it'll be for you

When you land, you need to go through immigration, collect your bags (bags can't be through-checked between airports), clear immigration, then get to the bus or train or tube.

In terms of how to get from one to the other, you have four options:

• National Express bus - coaches every 30-60 minutes, journey time of about an hour
• Tube (Piccadilly line) to Kings Cross St Pancras, train to Luton Airport Parkway, then bus up the hill to the airport - 1 hour for tube then 30-60 minutes for train+bus
• Train (Heathrow Express) to Paddington, tube to Kings Cross St Pancras, train to Luton Airport Parkway, then bus up the hill to the airport - 40 minutes to Kings Cross, then 30-60 mins for train+bus
• Pre-booked minicab - 40-45 minutes drive assuming no bad traffic

Given that, I'd suggest the bus, though the minicab option is there if you don't mind paying a lot more

If you're an EU passport holder, you can probably get from the plane to the airport exit in 30-40 minutes. Then you need to get to the Central Bus Station in the middle of Terminals T2 and T3, that's a 5-10 minute walk from T2 and T3, or about 15 minutes from T4 or T5 via tube/train transfer. Next, you need to wait for the bus. Then about an hour on the bus to Luton, then checkin again there.

Don't take a black cab, the price will be crazy. You could pre-book a minicab or similar, but it'll cost many times more than the bus, and won't save that much time. If traffic on the M25/M1 is bad, you'd miss the onward flight either way, and if not the bus should get you there. If you're worried about a 10-15 minute difference, then you shouldn't be booking tickets at just above the MCT, and need to pick a safer itinerary!

Either way, that journey is most of your transit time there, so any noticable delay (flight, immigration, traffic) and you'll miss the checkin deadline for the onward flight. On two tickets you're screwed at this point, on one ticket you'll have to get re-booked and wait for the next available flight

This is not a job for public transport. All transport options involve going into the city and back out again. Both airports are outside the M25 on the same side!

Arrange for a taxi, it will cost around £50. You can pre-book one online to have it there waiting for you.

It can be done. You will be at the mercy of the traffic. As someone who has lived in London my entire life, I would not book these tickets. The money saved by booking a cheaper flight from Luton is unlikely to outweigh the additional stress it will cause.

*edit*

It has been pointed out that National Express coach does go directly around the M25, which would make it the only viable public transport option. The price is £22.50 (correct Dec 2015). An independent travellers guide has a dedicated page to this coach journey, and provides this statement:

"The absolute minimum time with no contingency between flights that you should contemplate making a transfer between Heathrow and Luton is 4.5 hours."

The official minimum connect time between Heathrow and Luton airports is 3 hours 25 minutes. This is the shortest time the airlines will allow if you book connecting flights on one ticket.

It suggests your timing is tight but usually doable.

Personally I think if this went smoothly, if you have an electronic EU passport, no bags and a reliable taxi firm waiting, you could be airside again at Luton in less than 90 minutes. But things do go wrong with air travel and that two hour contingency can disappear fast.

I suspect you are connecting across separate tickets because Luton is a low cost airport. Therefore you are taking the risk of being stuck there or having to buy a replacement ticket. If you are happy with that, book the tickets. If not, find another route.

A lot of pessimists here. You'll make it 95% of the time using a cab. It just depends on how serious the 5% consequences are.