The best charge is a free charge of course.
The Waitrose & Asda charging posts are run by Polar (now BP Pulse) so if you have their card anyway they will be free to use, they appear on your monthly billing but at a zero rate. I do use these posts if I’m shopping there but don’t make a special journey to use them, so, 20-25 miles worth per hour (7kW), maybe £1 worth of electricity.
Train station car parks often have PodPoint charging posts which are free, but it costs a fortune to park, but, you can get a Blue Badge concession for many of these car parks which makes for free charge & free parking. Currently these car parks are near empty due to much reduced passenger numbers.
Dealership chargers are often ICE’d or the dealers are charging their demo cars. However, my car dealer said it is fine for me to plug my car in & go do my shopping across the road at Sainsburys whenever I want (I never have).
Dealers vary, some will let you charge, some don’t, if you need to charge then no harm in asking.
Often when a new rapid charger is installed, the provider will provide free charging for 1 or 2 weeks, if you can get a free charge then do so, if you don’t usually need to use a public charger then this is the time to find out how to use them.