I have not seen yo yo for years now and I don't believe you can get them, but the viscount ones are around. We have a local store (Lidl - I think thats the correct spelling, it's a german store, in the UK) that sells them.
The 2 differences between them is that Viscount is made by McVitie/United biscuits & Yo Yos were made by Burton biscuits. Also, I'm pretty sure that Viscounts were made with real milk chocolate, whereas the coating on Yo Yos was merely 'chocolate flavoured'.