I submitted my renewal about 5 months before expiry (last year) via the TVP online forms.
Remember to save a PDF copy of the completed form and any accompanying documentation. When the guy came to do the home visit, he didn't have everything with him, so good job I did.
Also, if you have acquired or disposed of anything previously, its helpful to have kept copies of the notification you should have sent them at the time. Too late if you haven't, and hopefully you wont need them.
During my home visit they checked my firearms etc. and then asked where a certain one was. I had sold it about 18months previous and notified them via their online system as requested. They said their records didn't show anything, but I produced a copy of the notification I submitted on their online system which kept them happy.
This isnt the first time they have lost a notification from me.