Does your insurance say you need it ? NO !!
Does your MOT say you need it ? NO !!
If the idea of having race seats turns you into an overnight racing driver then when you do have an accident, the chances of it being a bigger one are increased.
A cage alone wont save a head or neck injury, it is designed to save the car from causing you further injury.
So caged or not if you have a big enough accident you will get hurt.