If you're only doing 12 miles a day plugging in at work on the granny charger will be all you need. I wouldn't recommend charging fully from the granny every day but if you are only needing to add a few miles worth of charge each day I would think that's fine.
I have a wall charger at home, wouldn't think of not having it as often so longer runs where the car needs fully charging but I also use the grant charger at work a couple of times a week, it's free and I need the charge to get home.
Cost wise I'm with octopus on the 5p for 4 hours each night. An empty battery needs about 30kwh, more of its a new battery and completely empty, but at 5p per kWh it doesn't cost much.
For the ID3 VW are recommending charging to 80% and discharging to 40% if that's all you need each day, to help the battery life. Many on here will say just plug it in and use it, but there are plenty of others, myself included, that wouldn't charge to 100% every day if you don't need to.
We have done 53000 miles in our Golf over the last 3 years so often have to charge to 100% but when we're only doing short miles I only add what I need to keep the battery above 50%.