Isn’t that what my flight plan is for?
There is no ARO/AIS in UK and most small airfields don’t receive flight plans (I think they can setup AFPEX acess and fetch them but the majority don’t as it seems NATS used to historically charge for it while most GA airfields are poor as hell and have PPR anyway)
Booking out is not related to flight plans.
Yesterday, departing from EGTE (to EGKA) a phone call was made to ATC to book me out (made by the flying school which took the £70 fee). No flight plan.
Why?
No idea. It has no obvious value, because you will depart when you depart, not before and not after, and until you depart ATC has no action to take 
For flying an ILS for training without landing, “booking out” make sure they have your details to send an invoice…if you call on empty frequency out of the blue to fly an ILS, you get told told to “book out” as they will need to get your personal and payment details
It seems lot of it has to do with administrative items unrelated to flight plans, like paying a landing fee?
Just guessing…I think there are smart ways to surtout the admin part of things (like sending an invoice to reg owner and letting him sort it out) but hey it’s another UK quirk
CAA used the practice of “booking out” to sort the gap in ICAO flight plans plus the lack of ARO, see Cap694
In some places, you end up sending an actual flight plan (Eurocontrol IFR or international), get PPR from operations or owner by email/phone, as well as getting book out by phone with ATC…this was the case for my flight to Oxford last month, not lot of hassle really (you may also get asked on top to “pass your details” by RT when you speak to approaches while flying if you end up under FL80 during cruise )
As a foreigner in the UK it confuses me too. If I had to guess, it is a throwback to some quasi military airfield operational structure? Is the operations room (ops) the same? For example, “let me head over to ops and book out” seems very military. Being based at White Waltham, we were asked to book out, which I did out of politeness, but as a aircraft based there there were no fees due. It was a bit of an irritation, with an extra stop to make if you were in a rush.
At military fields I have visited (Canadian), they had an ops room keeping track of everything going on…
In contrast, at my home airfield in Canada, I used my remote fob to open the airfield gate, drive my car to the hangar, pull out the plane and leave. If it is within Tower operational times, and a flight plan has been lodged, they will open the plan a note the time of departure; without me doing anything!
Yes, much of this sort of stuff does have its roots in military practice. Like anyone who’s been indoctrinated into a certain way of doing things, military people tend to try to replicate their old world in their new world. Much of it is totally unnecessary, but it brings comfort to those who struggle to accept the idea of people just doing their own thing without some sort of system controlling them.
I believe booking out became widespread after an accident somewhere in the south of England, Hampshire I believe. I forget how long ago it was, but basically a chap (or two chaps, I forget) took off in a Cub (or similar) in poor visibility without telling anyone where they were going or leaving any clues. They weren’t missed for a few days, and then when the alarm was eventually raised no-one could find any trace. The aircraft had actually crashed in dense forest with no witnesses, and it was only when the leaves fell from the trees in the autumn that the wreckage became semi-visible from a footpath some distance away and it was found.
There was then some recommendation that airfields should log details of departures in order to assist with searching for missing aircraft. Of course it wouldn’t have kept anyone alive, but the wreckage might have been found sooner. Airfields up and down the land have been applying it with their customary zeal for information gathering ever since.
