I would add that my 'holiday' seats were on regular scheduled flights with TUI with passengers booking as individuals just for the flights, and not even just as a 'flight only' option with a tour operator.

Each and every company is different. The best advice is to ask. Genuine reasons for not being able to move include seats being blocked for C of G, reserved for crew rest and seats where an additional charge is levied. Unfortunately, some cabin crew can be rather abrupt, rude and totally incorrect in their explanation, but right or wrong you have to do as they say. If I am asked and I have time I will always check to see how many people can move from one section to another, the answer is generally that it is OK.

You can see the effect of some of these inaccurate on posters to this thread. For example, many aircraft have seating zones for C of G calculations. If you move within the zone, you will not affect the C of G. A well known operator has a standard loading plan. This involves certain rows being blocked depending on passenger numbers. Providing the blocked rows are not sat in, people can sit wherever they want. Other loading systems generate trim settings for take-off. With these you can typically go wherever you want after departure. Moving back to your assigned seat for landing is typically not required. Others require a certain (aft) trim setting to obtain minimum drag (to avoid a tech. stop), therefore moving forwards is not OK. On public transport aircraft there is a reasonable margin beyond the published limits to allow for the weight ranges that make up averages and the occasional minor mistakes. Not surprisingly these are not always published.

Can I ask a dumb question? How many passengers would have to move seats to cause a C of G / trim / etc issue? Not many on a very small aircraft I know but I mean your average commercial aircraft.

Good question. I've just checked some blank load sheets (E175 & E190, so smallish aircraft) and can see that if you were loaded at the aft C of G for landing, on an E175 moving five passengers from the front to the back would put you outside the published loading envelope. Approximately 10 passengers would have to move on E190 to achieve the same effect. The effect on takeoff is less because the C of G moves aft as fuel is burnt. One hour's fuel is equal to about two passengers sitting in the front. And in case you are wondering, the fuller the aircraft, the more likely the C of G is to be in the centre if the envelope. This is due to the design of the aircraft. Your problems start when you fly just a handful of passengers with unusual freight in the holds. But given that we are aware of C of G problems when lightly loaded, we pay close attention to the initial C of G and "blocked" rows.

Thank you very much for that, PM.

On one flight, before departure, a male passenger swapped with MiL so she was near us. He made the offer and the FAs were not consulted.

The problem I find (particularly with KLM for some reason but it might just be a coincidence or a lot of inexperienced flyers) is people sitting in the wrong seats. Its a right hassle for the FAs to sort it out and often simpler to take their seat.

I can remember once being on a 737 when the operator had loaded all the passengers at the back leaving about 2/3 of the seats empty - after some consulation the Capt asked that people "redistribute themselves forward please" as he was worried about CoG issues. I suspect it was more likely to be handling he was worried about than tipping the plane over

Dash400? I've seen them move or deny people the request to move for trim reasons

If there is nothing in the rear hold on the Q400 it can get very limiting to the point a line check can't be done because the jump seat needs to be empty.

Be sure to let cabin staff know when you change your seat if you've ordered a special meal, as they will be looking for you at your assigned seat to serve it to you.