If you are installing a large system (your inverter has an output greater than 3.6kW), you need to apply to connect to the national grid (via Electricity North West). This is to ensure that if you are exporting 100% of your electricity at any particular time, the network in your local area can cope with the additional electricity. Your installer should be able to apply on your behalf. Unfortunately, there is often a wait of several weeks before permissions are granted.
There is a possibility that you may be charged for any upgrade works to the grid that are required to enable your electricity to be exported. If this is the case, you can avoid this by asking your installer to set a restriction on the amount of electricity that can be exported, so that it is below the threshold where permission (and the associated network upgrades) are required.
If you have a small system with an inverter with output 3.6kW or less, you do not need permission in advance, but you should notify Electricity North West once the installation is complete. Again, your installer should do this on your behalf.