Mobile data Network selection

In settings > mobile data > network selection, there is an option called automatic. I turned this off, and it seemed to show which other mobile networks there were available, e.g. EE, 3, and so on. I stayed on my network and did not connect to any of the others. I was wondering what would happen if I chose to connect to one of the other networks shown?

Try it & see.

Probably nothing will happen unless they have a partnership agreement with your own network - which usually applies more to roaming, abroad & would automatically switch if you just left your Network set to Automatic.
If you have no contract with the other network, they will not feel obliged to connect you.