What if you don't have any USD? You can also instead by 10 futures contracts and make 0.50 GBP.
Additionally futures aren't just for "betting" they are also for offloading risk. For example a company might expect an sale in another currency and might want to "convert" that earlier to offload the risk of making another purchase in a different currency.
I guess I'm assuming you can always borrow USD if you want to make this type of trade on the spot market without actually owning USD in the first place, or if you want leverage.
Additionally futures aren't just for "betting" they are also for offloading risk. For example a company might expect an sale in another currency and might want to "convert" that earlier to offload the risk of making another purchase in a different currency.