ISTR seeing PoWs dressed in British battledress, but with a large circular, dark coloured patch, in the back.

Correct. I think it was a chocolatey brown coloured patch IIRC.

My Mum and her sister used to walk right through the middle of a POW camp here in West London on their way to school, and many of her school friends used to stop and talk to the prisioners through the fence, even though, they were not supposed to. It was the Italians who used to chat up da laydies 😀
A number of her school friends ended up actually marrying some of the Italian POW’s after the war ended. The camp was built either side of the A40 by the old Aladdin factory, a few miles east of RAF Northolt, and the POW’s were used to build the Central Line extentsion after the war ended, Greenford and I think Perivale tube stations were built by POW’s.
A lot of the camp, including some of the buildings and that main through path, were still there back in the early 1970’s when I was a teenager, and us kids used to play on the site all the time.