Hi,
When I started converting machines as a kid of 16 in the 1950's, everything in the U.K. was Mills, my late father operated a few Jennings Bronze Chiefs and the likes, It was not until 1960 that the Jennings Tic-Tac-Toe, Club Chiefs and then the Jennings Governor took over, mainly because of the British distributor Coughtrey's Automatics Ltd, who in 1962 became the largest distributor of Jennings in the World, George Coughtry was an old time operator who was fondly known as the Governor (see attached add) the Jennings Governor was named after him.
Interestingly on the south side of England the operators operated mainly a Jennings Governor and a Keeney Upright in their club locations, while in the north of England this combination was taken over by the Bally Treble Chance Upright and a Sega Mad Money models, but was quickly replaced in 1963 by a Bally Money Honey and a Bally Gold Cup upright models. eventually just like the Las Vegas casino market Bally totally dominated both the British Club Market as well as the new pub locations where the Bally Jolly Tavernor and Bally Sir Prize was the first choice for operators.
The early 1960's was a Bonanza for the British operators, today the entire industry is dominated by Scientific Games (A company that was first owned by Bally and banned from operating in the U.K. in the 1960's) who are operating FOBT (Fixed Odds Betting Terminals) in the betting shops that are on vertualy every corner in every Town in the U.K. and by either the Gauselmann Group or Novamatic two giant companies who have bought up most of the British manufacturing and operating companies and dominate the Pub market.
So to answer your question about Watling items, there were never many Watling machines of the 1930's operated in the U.K the main European market for Watling was France, in fact most of the Watling Rol-A-Top's that you see offered for sale "Brand New" and still in their crates came from France, they were discovered in a French customs warehouse by a movie company that was making the movie Borsoleno in Marseille where they were used as props, they were later bought by the Paris collector Bernard Dewitt, and sold on to several collectors in the early 1980's.
I can tell you that the original Watling tooling was sold to Bell-Fruit Manufacturing Ltd in the U.K. they produced the mechanisms for their first slot machines called "Three-A-Like" in the mid 1960's, if you go to
http://www.coin-opcommunity.co.uk and in the search box type Freddy Bailey you can read the article I wrote about the connection between Watling and Bell-Fruit along with more than 25 articles about the industry that I have written for that trade Magazine.
Freddy Bailey