If four adults were travelling together, the prices would be about the same.
A taxi going from London to Brighton costs roughly £100 (£25 each).
If you had fewer passengers, the taxi would still cost £100. It would only be the same price and slightly more convenient if you were going as a small group.
This observation doesn't account for rail cards and discount codes.
Trains move a lot faster than cars and they sometimes give you cheaper return tickets, however some Uber cars can take more than 4 passengers, some can take 6 or 7.
