Normally on the way down Glasgow to Luton then you would be at FL350, FL370 or FL390, usuually.
On the way up, abeam Manchester then you would "normally" be about FL340, so I think on this day the plane must have been quite empty. Sometimes due traffic you would fly North cruising at FL350, so maybe he said 35 & you heard 39?? Who knows.
Hope this helps.