There are some companies such as https://www.worldspreads.com/ that allows you to bet directly on the spread.
If you trade shares, it can be very money-saving to bet on if the the price moves up or down. You don't need money to buy the shares to go short or long. Instead, you bet directly on the price move as little money as you wish. There are no broker commissions, executions are fast, guaranteed stops, etc. are some of the benefits of betting directly on the price move.
You can bet on share prices, index movements, etc.
In the word of Forex, however, there might not be much point using spread betting as technically there is no real difference between spread betting than executing Forex trades on a margin with the brokers.
Maybe, the only main difference that when you just begin to trade Forex, it might be hard to follow the idea that you go "short in USD, long in EUR" at the same time when you buy EUR. The concept of "betting on if the the price moves up or down" is much simpler to understand.