My perception overall is that demand in this game is much higher than in the real world. I think one reason it may seem lower to you is your location - LGW. Real-life pax. numbers in the UK tend to be heavily skewed towards London, because historically that's where many of the flights were. I'm sure the same is true in other countries too. I don't live in the UK any more but I know many people either travel to London or via other European hubs to catch their flight, as the range of destinations just isn't available regionally. This probably leads to places like Gatwick having higher numbers in the real world than their 'logical' coverage might suggest.
As an example, in my last big gameworld I was in Turkey, flying out of six airports. By late in the game I was flying just over TWO HUNDRED flights per DAY from those six destinations to the UK! That's an even mix of A320 and A321 at 90% lf so around 34k per day each way, or 13M per year. Bear in mind that I had only 50% of the share in IST and about 60% to 70% elsewhere. So, in reality there were probably at least 20M to 24M pax. each way per year between the UK and Turkey. I'm sure that is quite a bit above the real life figures. Most of those flights were NOT from London - I was serving 27 UK destinations at least daily and most with double-daily A321. In real life there ar probably only about 12 to 13 places from which you can fly UK-Turkey and most of those will be 2 or 3 times per week. I had 16 A318s per day from LCY to Turkey! I bet in real life two thirds of those pax. would have departed Gatwick. 32 daily A321 from BOH, SOU, EXT and SEN to Turkey in total - all Gatwick irl.
I know this is just one example, but I've noticed a similar pattern in previous games (I once had bases in LPL and BRS with about 30 long-haul A330 flights per day from each - Japan, China, HKG, India, South America as well as US and Canada. This demand just doesn't exist in the real world). The point , I suppose, that it doesn't really matter as long as there is a balance that makes the game playable - not too easy and not too hard/boring.