With an Uber Taxi, it gets cheaper per person as the passengers increase up to 4 passengers, then the price goes up because you need a more expensive 6 seater Uber. But the train is cheaper in every metric.
With 3 or more Adults, each person is paying just £6.05 because of Group save supplied by the c2c train line.
With 4 passengers, an Uber ride from barking station costs just £15 per person because the taxi costs £60 regardless of the number of passengers. However, one passenger is only paying £9.10 on C2C so it is cheaper by train in every way. If your just one passenger or 6, it is cheaper than the cheapest configuration on Uber taxi although is gets close with 4 passengers.
The Uber is slightly more convenient because you can chose the pickup and dropoff point and with 4 passengers, you're only paying £8.95 more than the train.
The train is cheaper still when you add return tickets and rail cards.