If your system is larger than 3.68kWp, 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.
There is a relaxation to this rule if you are also installing a battery. The system limit increases to 7.36kWp, but with the same export limit of 3.68kWp.
If your system is smaller than 3.68kWp, 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.