depends what you mean by 'widespread'; in halfords? -no. In cycling? -yes!
Cantis were in common use on lightweights when I started cycling in the 1970s. In fact they were a popular brake because the bosses could be added to a 'gappy' frame (*) which made for a really versatile bike with good brakes. They were de rigeur for CX, the best touring bikes, tandems, you name it. To give you an idea, it was about as likely that you would see a Mafac CP brake as a canti in many cycling clubs at that time.
(*) Which was most frames built in the UK since the mid 1950s. The fashion for close-clearance racing frames was pretty new; IIRC campag didn't start to make their piccolo brake until the early 1970s, and in fact side pulls of any kind only became popular for racing since ~1969 when the (standard reach) campag brake was first introduced. Before then it was mainly CPs, dodgy universals, or cantis. If you bought a dedicated racing frame of continental design it might have a brake drop of ~57mm (and mafac CPs probably), but most UK-built frames had bigger clearances than that. Many secondhand (as well as new) 'lightweight' road frames used the longest reach centre-pull or side pull brakes to reach to 27" rims; all pretty poor. If you wanted to run sprints and tubs you needed to find another 4mm brake drop and this meant that the brakes were guaranteed to be extra-feeble. If you started out with a used frame (that didn't have canti bosses on it already) it made lots of sense to have canti bosses put on it when it was being resprayed; lots of my chums did exactly that.
CLB used their own boss design but Mafac was most popular and this became the one that was copied and used by the Japanese. I dunno when (say) Dia Compe started to make cantis (they had been making brakes since the 1930s and had been involved with Weinmann since the 1960s) but the first mass-produced MTBs (eg the spesh stumpjumper) used Mafac cantis because they were the best (and most popular) then available.
five different models of MAFAC canti, circa 1964
Jacques Anquetil, ~1957Of course cantis had been popular well before MAFAC even existed; for example since 1929 the Resilion cantilever brake had been used on quality machines in the UK, and was arguably one of the first really good bicycle brakes of any kind.
1949 Resilion brakescheers