A welcome as well from me, enapace. It would be remiss of me not to mention that we also have a very comprehensive help guide for ITA Matrix in our Travel Tools forum, which has the advantage over YouTube of being able to post queries and get responses from a set of experts.

ITA Software Matrix Airfare Search Consolidated Information and Help Thread

As you look around this forum, and further into the site, you'll find yourself leaning quite heavily on routing tools such as Matrix to build complex itineraries for things like Tier Point-efficient routings and starting points outside the UK mainland that can result in significant cost savings on premium cabin fares. If you're prepared to go to Dublin, you should possibly be thinking about starting your trip there and coming back through London to drive some big cost savings - lots of detail on what are generally known as ex-EU itineraries in this forum

I'll agree with others that the myriad of options for entering the US means that the advantages of pre-clearance are rapidly diminishing, particularly with the introduction of Mobile Passport Control and the associated app, along with Global Entrg for those who - like me - are prepared to pay for it. Certainly it was helpful yesterday when a late arrival meant my 95 minute connection had become 60 minutes - even with having to see an officer because I was entering on my current passport for the first time, and printing a BP at the Flagship check-in area, I was still airside again at JFK T8 just 12 minutes after leaving the plane.

So yes, you're right: LCY is a lovely, compact airport - but more so for arrivals. The same might be said of EWR where passport control is generally very good, but departing not such a great experience.