Tipping a policeman

I’m indulging myself during this tedious lockdown by re-reading Arnold Bennett’s Imperial Palace (1930). At the moment I’m wondering about something that occurs in the episode where dynamic Gracie Savott parks her car outside Smithfield market, and asks a policman to keep an eye on it.
On leaving the market, ‘she resumed her dark cloak, tipped the policeman before Evelyn could do so, and slowly climbed into the car’.
I’ve come across the custom of tipping policeman before in novels of the twenties, especially detective novels.
These days, though, I’ve feeling that I’d be regarded with some suspicion if I offered our local community police officer a fiver for services rendered. So – does anyone know when the custom of tipping policemen ended? My guess would be somewhere about the Second World War. Am I right?