Okay. I can understand why you might wish you were staying! However, for packing, it will make things a lot easier because the trips have different needs.
Jet lag is a real thing! It affects different people differently, of course.
I've just returned from my trip to the UK which involved a 4 am waking, 2 hour ferry, transit to airport, several hours' wait, and 9 hour flight across multiple time zones. And I am happy to say I suffered almost NO jet lag. A first for me, flying east. I have no trouble flying west. This article is quite interesting on that subject, and possibly helpful for planning.
Here's what I did, that helped. For three days before my flight, I woke up one hour early each day and tried to go to bed one hour earlier. When I woke, I immediately went outside into the daylight without sunglasses for 40 minutes to an hour, then I remained in bright light for the morning, through the mid-afternoon (wearing sunglasses later on). I also drank coffee in the mornings, but stopped before noon. I took no sleep aids or melatonin, but some people say taking melatonin helps at night. It doesn't agree with me so I opted not to take.
About an hour into our flight (which left late afternoon from Vancouver), I put on a eye mask to darken my surroundings. I find regular neck pillows worse than useless, so I splurged on this thing -- a Turtl travel pillow -- and wore that, plus my headphones on noise cancellation or on my audiobook. We had a lot of screaming kids on our flight sitting nearby. Did I sleep? Not really. But I rested in the dark for bout 7 hours, and dozed for about an hour or 90 minutes during the flight. About an hour before landing, I got freshened up a bit. I also drank a lot of water. I didn't even eat the first meal -- I wan't hungry when they came with it and I decided resting was better for me.
I won't pretend I wasn't tired, but I got through an entire day in London, managed some tricky transit switches, got to the place we were staying, settled in, explored the neighbourhood, went for a walk through a local park, enjoyed a good lunch and a delicious dinner, and felt perfectly fine throughout. Spending time outdoors was key. I also had an espresso at lunch!
I slept well, woke early, and felt fine the next day.
There are apps you can get or sites you can consult for a plan. (I
Mr. Suz and I never buy business class because, $$. But it's undeniably better, if you can afford! If not, try to select your seats, if you can. I prefer a window, he prefers an aisle. So we often have a stranger between us. Such is life! (I like the window so I can lean my head; he likes the aisle because he needs to use the facilities).
Wherever you are sitting, DO try to get up as often as you can to stretch, walk a bit, keep the circulation going. Avoid alcohol. Drink a lot of water. Eat light.
It will be a little trickier for you to make use of natural light because it will be autumn, not spring, but in that case you can try a light box (the kind people use for SAD) or just bright artificial light. I have to admit, I tried this once for a trip overseas in February -- to less successful results.
Pack your carry on carefully and ensure it (or a portion of it) will fit under your seat so you will have access to things you need. More than that, realize that it can be very difficult to get at your under-the-seat carry on and open it up and pull out what you need, so keep organized there. Things I always bring in my backpack/ personal item:
Passport
Face masks
tissues
Computer
Kindle/ipad
phone
earbuds
Something warm -- whether that be a compact packable puffer (what I did this trip) or a blanket. The airplane blankets are not always great and flights can be insanely cold. I wore my puffer on my last trip.
Pillow (of some kind)
Small moisturizer, sanitizing wipes, saline eye drops. (Flights can be dirty and dry.)
Something to suck on for take-off and landing. (Prevents your ears from popping).
Sunglasses.
Notebook.
Scarf
Seconding Ms. Mary's idea about tights. You could also bring Heattech layers, really thin ones, or silk or merino, as Sal suggested. I strongly recommend merino layers for the Scotland/ Iceland journey.
