If an EU citizen arrives to the UK, would there be any difference in relation to passport queue length if different arrival methods (i.e. transportation) were used? For example, would the passport queue be typically longer at the airport than at the land border or sea port? Or does it entirely depend on the season and not on the means of transportation?
I have used planes, trains, ferries as foot passenger and as car passenger.
In each case it mostly depended on whether you were the first in the line or behind a lot of others.
For the trains you go through passport control before you get on the train. In that case you have to be there about 40 minutes before the train leaves, before that the passport line opens and starts moving. How long before depends on the location as well as on the staff there and whether the train is already in or late in arriving.
Be at the start of the queue and you are through in seconds, be a little late to join and you may be half an hour in waiting.
With cars going onto or off ferries it is a lot like that, it depends on whether you are lucky or unlucky in your timing. I think your checks are mostly when you get off or at both ends, if so, luck in where you are in the waiting line to get on and whether your deck and your line in it is the first or last to leave the ferry will determine the wait.
The one time my brothers traveled on Boxing day and only 7 cars used that ferry, all of them were through the checks fast.
Foot passenger, be one of the first of, walk fast and you will be early in the queue. Be one of the last and you have to wait for your turn.
In flights it can be that simple or your plane can land just after some others and it can take a long time even when you are the first off.
Of course the season does always have some influence. In the peak times it is more likely that you have a long well filled train, a busy ferry or even several arriving close together if you are in a port where several come in. In airports it is more likely that several planes land at the same time. But even at the busiest times it can be quiet when you arrive, just before a lot of others.
And even at the quiet times you can be held up when they want to truly check the people ahead of you.
And it depends also on whether the border services have adjusted the number of staff for the season and expected crowds.
One sample, AMS (Amsterdam airport, so not the UK but it is not that different in how it usually works,) most times I have had a few people in front of me, waiting time seconds to minutes. But one time I was unlucky and the automated system had broken down, waiting time over an hour, and everybody, regardless of passports, was much later than they usually would have been. And it was a day when it should not have been busy.
I am EU European and usually travel with my ID card as well as my passport and use the passport in the automated system when that seems to be a shorter wait (which is mostly the case.)