What alternatives should I consider to a used Citroën Berlingo estate?
If you happen to work for Addison Lee, then you might want to look for a Ford Galaxy. Not only taxi drivers will appreciate the Galaxy's enjoyable driving experience, nor its fine engines or third row seats that are spacious enough to take actual people. It doesn't have any clever storage solutions inside, though, and misses out on sliding rear doors, too. There are plenty of them around, though, so finding a tidy one should be easy.
Alternatively, try a Volkswagen Sharan or its virtually identical sibling, the Seat Alhambra. Both of them are huge inside, have well-appointed interiors and the benefit of sliding rear doors that should prevent your passengers from opening them on a car parked next to you.