I know this will get shot down as I have no idea if nationwide but I was in Tesco in Brooklands, (Barnes Wallis Dr, Weybridge KT13 0XF) , tonight and noticed a lot of their mobile phone cases and items reduced. They had items for various phones but mainly Samsung and iPhone.
There were phone cases from 50p.
Probably the best deal was this genuine leather notebook style case with detachable tassel for the iPhone 7.
It was £16 and was reduced to just £1.50!
Lots of other items including power banks reduced to £2
This was reduced to £1.
This reduced to £1.50.
There was lots more so might be worth a look if you happen to be in Tesco.
I found all these in the normal mobile phone accessories section.