Yes; if you fly abroad then the planning takes longer, sometimes much longer because you need to contact the airports etc.
But comparing to driving or taking trains/buses etc, I think it is still a fair comparison. You will spend ages sorting out trains for example, for a long run across Europe.
Taking an airline is the only way to do no planning at all. Just buy the ticket online, call a taxi, and turn up at the airport with your passport.
I'm sure that even you can't plan a flight to, say, Croatia within ten minutes from absolutely zero to startup.
It depends on the destination airport, and you picked one of the easiest places in Europe
LDPL LDLO LDSB LDSP LDDU... all easy.
In this particular case it is about 1-2 hours to get the whole "routepack" knocked up and printed, and then I wait to hear back a confirmation of avgas, opening hours, etc. In practice, for such a trip, the actual planning time is not an issue because you need to pack a load of stuff, etc. anyway.
But that's IFR. VFR would take longer, with the charts all over the lounge floor and crawling all over them. Best part of a day.
But one would not pretend there is utility value in flying UK to Croatia
It is just a really scenic trip. At the top end of the market (a TBM850 or a jet) you can do it efficiently, but at a huge cost.