This is a really interesting thread because I fly sufficiently often that I don't think about it any more, so it's forcing me to think.
The plane stops at the beginning of the runway and then accelerates harder than most car drivers. But when actually taking off you barely notice anything. It's not like a rollercoaster. More like a department store lift.
The "fasten seat belts" sign will stay on for the first 10 minutes or so of the flight, while the plane is climbing. But after only about 10 seconds of the flight you will hear a little "bong". This is the captain telling the cabin crew that the takeoff was successful, so they can stop being prepared for a possible emergency evacuation and start getting ready for the in-flight service.
When you're up in the air it's like being in a train carriage, but a little bit more cramped. There's a constant whooshing sound, which varies in volume depending on where you're sitting (it's quieter in front of the wings).
You can take your seat belt off when the plane is in level flight, but it's best to keep it on. Sudden turbulence is very rare (which is why they allow you to get up and walk to the loo) but it's just safer to keep it on. It's only a lap belt and you won't notice it.
During the flight the cabin crew will come round selling drinks and snacks, which you will probably only be able to pay for by card. You can take your own food on board. If the UK has sorted out its liquids policy by May 2025 you may also be able to take your own drinks brought from the supermarket.
There is a call button to attract the attention of the cabin crew, but that should really only be used if something urgent happens. Don't ring it five minutes into the flight and ask for a priority gin & tonic.
On the subject of G&Ts, it is widely believed that the effects of alcohol at altitude are about double what they are on the ground. So a 50ml miniature of gin will pack the apparent punch of a UK pub triple. Don't drink alcohol if you need to drive a hire car (or your own car home on the return journey).
You are very unlikely to feel sick. This was a problem in the 1950s and 1960s with the old "clunga-clunga-clunga" propellor aircraft, but not with modern jets. Airlines have mostly stopped providing barf bags to everyone because they just sit in the pocket in the back of your seat and get torn for when someone finally uses one. But if you see anyone using one, it's because they had too many pre-flight brandies. There is no need to take seasickness pills.
When the plane is descending your ears and sinuses may hurt a little bit from the pressure change. This stops as soon as you land. You might be a bit deaf for half an hour afterwards as your ears balance up. If you have a cold or sinus problem when flying, ask at the chemists for a decongestant.
When the plane lands, the passengers may or may not applaud.
When you walk off the plane, there might be a tunnel thingy that takes you directly into the terminal, or you might arrive on the "apron" (the open area of the airport). In the latter case you might have someone telling you where to walk to, or there might be a bus.
When you get to the terminal the first thing you will encounter is passport control. If you're travelling to the EU in May 2025 this will be one of the new EES terminals, which will take your photo and your fingerprints. Nobody has used one yet (they are being introduced in November 2024), so we're awaiting news on the experience.
After that you will go through a pair of automatic doors that say "No return after this point". This is where you leave the security area.
Next you get to baggage claim. You may have seen this in films. You find the conveyor for your flight and wait until your bags come out. Once you have all of them, you walk through the customs green channel. If you are flying to a European holiday airport there is about a one in a million chance that you will be stopped and checked.
Once you leave the baggage/customs hall, you are in the public area, known in the trade as "landside" (as opposed to "airside"). Here you will meet your rep if you booked a package, or someone with your name on a sign if you booked a transfer to your hotel, or you will find signs to the taxi rank or the buses.
Hope this helps, ask me anything, etc.