The SORN used to be an annual thing but recently it changed. Now once you SORN it you don't need to notify DVLA again.
Regarding the Historic Vehicle class now not needing an annual MOT test, while this is true, you are still required to maintain the vehicle in a roadworthy condition.
If you have any sense at all you'll continue to take it for a voluntary MOT test.
The crazy thing is that before they decided to waive the MOT test, the government consulted car clubs and other various motoring organisations for their input. They conducted reviews of the number of accidents and number of fatalities involving vehicles of this age and despite most organisations suggesting that it would be a bad idea to waive the MOT, they went ahead anyway stating that the number of deaths and serious accidents involving vehicles of this age was a tiny percentage of the overall carnage motorists experience.