Oi, I'm not slow! I did some odds compiling when I worked at the William Hill Regional Office in Stockport, it was more local events, like non league football or greyhound races and the bets would be restricted to regular punters and for a maximum liability of around £20 (it was over 40 years ago).
It's was incredibly difficult to price up ante post events in those days, if a stranger walked into a shop and wanted a bet like yours they'd be either refused, given the selection at lesser odds and/or the stake would be restricted.
For events like Miss World back in those days bets would be only laid to a stake of £10 or to win no more than £50 dependent on who the punter was.
Can you remember the name of your 100/1 placed horse?