We used to run just one family car, but we had a local Sixt branch that I could nip down to. Would get the cheapest Corsa “or equivalent” (often better) for about £20 a day and unlimited mileage on the times I was away riding for the weekend. Fine for hammering down the M4 with a bike in the back.
Wanted a bigger car anyway for camping, family biking, etc so we have our Stepwgn that does a few thousand miles a year, mostly family trips but it covers the times we need to get two kids somewhere different at the same time. Have an EV for the everyday stuff.
Zipcar and the like are OK if your usage is up to a few hours at a time, it gets pricey once you take it for full days or more. Some more central London friends do it anyway because it’s much more convenient to get one a street or two away rather than get to a hire place, particularly once you have kids.