Pretty sure that the PoW was in tropical rig. What's tropical about Northern France?
The Army seem to have the sensible practice - don't know if it's official or not - that if it's hot you wear the "hot climate" gear.
Many's the occasion I've sweltered in my No 1 at functions in UK or Germany where my Army colleagues were as suitable attired as PoW was.......
I do however recall one time many years ago at the secret Hampshire helicopter base, when there was a Dining In Night during a severe heatwave. Sensibly (unusual that!) a Tannoy went out "Tropical Mess kit optional!"
What was really sad - compared to now - is that 90% of the officers had the kit from a previous tour and wore it. At the time there were Sqns in Hong Kong, Singapore (2), Sharjah (moved there from Aden) and Cyprus; and SAR Flights at Muharraq and - in the then fairly recent past - also in Libya (pre-Gaddaffi) and British Honduras (pre-Independance when it became Belize).
O tempora o mores