Flew on that to Salzburg and back in 1994. On the way out we had a delay of 6 or 8 hours, variously due to Spanish ATC "working to rule", a nervous passenger having a nose bleed and deciding not to fly and the baggage handlers having to rummage through the hold for his bags (and through all the other bags to see if there was anything worth nicking! - my dad's suitcase was knifed).
Delayed on the way back too to the point they were switching off the displays in SZG as after a certain hour, the airport had to close so as not to disturb the locals. Just our aircraft and an Air UK 146-300 bound for Edinburgh on the apron. We got underway pretty late and I remember flying in over Kent on the way to LGW and seeing all the towns in Kent clearly illuminated with all the roads linking them up like fairy light chains. Pretty special for an 11yr old and some compensation for lots of sitting around in airports not so much "are we there yet?" but "are we going yet?"
According to Wikipedia, KLM bought its first 15% stake in Air UK in 1987, increasing it to 45% in 1995 an then buying it outright a couple of years after 'Leisure were sold to Unijet.
Their predecessor Air Anglia of course had establish strong Dutch links (services to AMS and a fleet of Fokkers).