Yes. The Stamp Swap still works. My DD sent some off about 6 weeks ago and had the equivalent barcoded ones returned within a week. I did the same with an old booklet of 12 first class after she told me to download the form. My new bar coded stamps were returned very quickly and Yes, it is a FreePost address.
The whole stamp-swap fiasco must have cost Royal Mail/The Post Office a fortune. One can only wonder why it was really necessary. Was it because they knew that The PO was soon to be sold to a foreigner; a Slovakian from memory? It probably wasn’t to make us all clear out our drawers!