Christopher, it only seems to be 'ONLY'. 😉
Since you fly in the UK, the Heathrow airport itself generates most international airlines of the world. So you have a tedious task to do, sir.
As I am mostly a heavymetal flightsimmer, most of my flights originate and/or end at large airports (80-90% of time in Europe). One of the basic things I need to feel immersion (sim-wise) is realistic traffic around.
One of the things VATSIM never gave me, as much as I love the real and diversified human interaction it is based on. So nowadays I'd rather fly offline (I don't believe I wrote it, lol).
As for traffic, I have been for years on custom AI traffic. I started compiling my own traffic in FS9, then FSX and now P3Dv4.
Maybe someone will find it valuable, so let me share some basics of my traffic creation workflow.
1. I regularly check the best AI repaint sites to download the latest repaints. Those sites are mainly juergenbaumbusch.de and jcai.dk, but also atco-repaints.com, kyles-ai-works.com and, yes, the AVSIM library (flyingcarpet75 used to be nice too).
2. I regularly check the best AI flightplan databases to download the latest flightplans. This is mainly unitedtrafficteam.com/index.php/ai-flight-plans (updated daily) but also AIG and, again, the AVSIM library.
3. Each flightplan set consists of 3 essential text format files: Aircraft.txt, Airports.txt. and Flightplans.txt. I edit the Aircraft.txt file with NotePad to replace the aircraft names with the ones I have in my Prepar3D_4\SimObjects\Airplanes folder and just save it.
4. Then I compile the flighplan using AIFPC (AI FP Compiler) - it's a very easy automated thing to do. And it ready!
5. I also often check my created traffic files (the bgl's from Scenery\World\Scenery - where they reside) with ACA2017 - if there are any errors, which happens from time to time, I correct them easily.
I don't know much about the payware traffic programmes but my custom traffic is very accurate and current. Most of my flightplans are now dated the summer 2018.
At this moment I have 354 airlines (just checked), pax, charter and cargo, mostly European but also some from all other continents. All the main players plus a number of regionals.
This way wherever I fly, I always have traffic buzzing around me. When/where possible, I sometimes play real world ATC from liveATC.net and - for me - the immersion is huge.
As for AI aircraft models I use a lot, currently it's 192 (!) of them, e.g. TFS, FAIB (a huge base of these!), FSP(XAI), UTT, AIA, AIG, AIM, OSP, HTAI, CIS, Raven, SBAI and others.
They have been improved with some nice freewae AI sounds available at the AVSIM library.
I am curious to read how others create their traffic.
If you have any questions about my way to do it, please feel free to ask.