Not just UK FIR, France too.
It wasn't just nice in the UK, it was beautiful all over France. Spent the day crossing Europe in a Yak-52, from a dawn start at Abbeville, south via fuel stops at Gueret and Villefranche, then straight over the Pyrennees at FL105 to Alicante. At the top we estimated the vis as 70 - 80 miles. Even at lower levels it was 40-50 mile plus. Airfields 10 miles away looked as if they were right on the wingtip. It was certainly a privileged view.
But the moment we crossed the mountains into Spain it was much wetter, lots of haze and a hefty cu layer from 6000 to 12000.