Darth, I remember this too. It was a slushy drink and it contained stuff that reacted to the acid in the tum and made inert fluffy stuff like cotton wool to fool you into thinking that you'd had lots to eat and so to stop. However, I can't recall a brand name (I doubt if the BEEB told us) and it was one of those ".. on supermarket shelves shortly .." type articles.
As for the demise of TW then the BBC site
http://www.bbc.co.uk/science/tw/
says it all, really. Raymond Baxter, WWII fighter pilot who hosted the show from its start in 1965 for 13 years was unceremoniously dumped by the BBC because they though that his engaging and authoritative style was too pompous for the "soar-away-super-seventies" generation. The programme suffered from constant meddling and in the end was far distant from its roots and was a banal showcase for BBC in house visual designers and set dressers. Its science base had been lost along the way as the BBC felt that the population were too stupid to understand complicated "how stuff works" ideas and the nanny mandarins found even more ways to squeeze snippets, trails, and snatches of its brain curdling East Enders into the schedules.