Can I use my bootloader-locked phone with a different network?
The bootloader being locked has nothing to do with whether it's SIM-locked to a particular network. It just means you can't flash a custom ROM to the device (without unlocking the bootloader). You don't need to unlock the bootloader to change SIM to a different network, but if it's SIM-locked to a particular network, you need to unlock it from that network.

Whether you can switch to a PAYG SIM is entirely up to the carrier, and nothing to do with Android. You should check with the carrier.