The sigwx chart for that day gives moderate icing and turbulence below FL180 in a band if weather stretching from the north of Iceland to the Midlands, with jetstreams at FL290 just south of Iceland and FL350 mid way between Iceland and Scotland. Although the chart forecasts moderate turbulence and icing below FL180 I would be very surprised if the vertical development simply stopped at FL180 and there may have been cloud tops much higher if this occurred at 20 minutes after departure.
I can't go into specifics, SLOP could have been used on the route (nothing to preclude its use technically) but I'm going to say either weather avoidance or being put on to a heading temporarily in order to climb clear of traffic, which happens a lot. SLOP isn't too relevant when you're under radar control, and even on the blue source routes it is rarely done. If it is then the turns are usually pretty gentle especially if the SLOP is only a mile.
The 22:50Z matar for EGPH that day shows CB reported, and the TAF for that evening forecasts CB so you could have flown overhead the field onto a left downwind for 24 or even flew through the localiser then onto a left downwind for 24
Either way, I'd put my money on saying it was weather avoidance or following ATC guidance.