Used reserve and collect option on Curry's website, and took my old white box into the Currys store.
Price was £19.99, and when I handed over my white box - reduced down to £9.99.
I was also allowed to keep the HDMI cable.
Can't see any issue returning back to store with another box, unless the same assistant is serving, and remembers me.
Given the lack of knowledge about the offer, it wouldn't surprise me if some customers were able to get two from the same staff member. I won't be trying it as it isn't in the spirit of the offer and given the recent unavailability of some apps on the white box, this feels like an ideal offer for those forced to upgrade.
If anyone is trying it, bear in mind that included passes are only good to be activated within a year of purchase.