It goes on to say that "[It was] originally developed in 1947, by Wilmot, Mansour & Company Ltd. of Southampton, it was first demonstrated to the modelling press in early 1948, and was available to the public in June 1948, when Aeromodeller featured Jetex power on its front cover. The first motors were the Jetex 100 and 200, with the more powerful Jetex 350 following in November 1948. The most popular motor, the Jetex 50, was introduced in May 1949, along with kits for a model plane and model car using Jetex power.
Jetex motors are powered by a solid pellet of guanidine nitrate, which burns to release a variety of gases in copious volume, leaving no solid residue or ash. Thrust developed is fairly modest, suitable for horizontally launched flying models rather than vertically launched rockets. The exhaust gas is not excessively hot, which confers a safety advantage. Fuel and wick to ignite the pellets was manufactured by Imperial Chemical Industries (ICI). The engine casing was made of an aluminium alloy and was reusable, new fuel pellets and ignition wick being a consumable that could be bought and used in the engine."
Since the motors release 'a variety of gases in copious volume', developed a 'fairly modest' thrust and the exhaust gas 'is not excessively hot', they seemed eminently suited for Meddings' purpose of disturbing the Fuller's Earth his model sets were sprinkled with. I would say that Meddings used the Jetex 50 motor for his purposes: it is small and was the most popular model which probably means it was widely available.
Another source of information both Wikipedia and Google pointed me to was the jetex.org website which is where I found this cutaway drawing and explanation of a Jetex 200 motor.