Re: Back in the distant past, before I worked for HP...
I'll admit, I admire your pedantry.
Heathrow's really close to me. There are several different routes there. The local minicab drivers know them all and all talk to each other so I'm likely to get there at that time of morning without too much issue, In contrast, getting to Luton involves the M25 or the North Circular, both of which can quickly descend into chaos. So you need to leave way more time to allow for this.
Heathrow's also pretty easy to get through security. They also have fast track lanes which I can use.
And most importantly, there are generally (but not always) more flights available from Heathrow. So where there might only be a 7AM (or earlier) flight from Luton or Gatwick, there will be several options between 7 & 9 at LHR, from different airlines, all heading to the same destination. So where I would need to get up at 4 (just as an example - don't hold me to it) to get a flight from Luton, I might be able to get up at 6 and still be able to get a flight from Heathrow to get me to my meeting on time.
I don't like getting up at 4 in the morning, especially if I have to travel then spend a couple of hours presenting. I'm not very good at doing that without adequate sleep.
My company's travel agent doesn't see this. They just see the "cheapest flight" policy and don't factor in the extra cost of cabs up to Luton and the fact that if I have to get up that early I don't have the energy to do my job so there's little point in me going anyway.
Besides, going back to your point, 40 mins extra in bed is quite a lot. But no, it's not as simple as that. But I think you know that.