Nextbase seems to be popular I have noticed while looking around they vary quite a bit price wise is there any benefit to buying one at £100 against one at £40
Mine is the 202 and was just under £80, so not the top spec one but certainly does the job. It's a clear image when played back so I'm happy enough with it. It comes on automatically, records automatically and has a G Sensor to record and save any shock or impact. As most do, it just keeps record looping in small chunks, so you have to do absolutely nothing, just get in and drive away.
As said, I regularly use our two cars so I purchased a second screen mount and wiring so it's just a simple matter of unclipping the camera and putting it in the other car. It takes seconds to do so not a chore.
You can view playback on the camera screen itself, remove the camera and use as a camera to take photo's of an incident so that all the evidence is on the same SD card, or put the SD card in your laptop and view all of the individual files.
Oh, and if you've got a bit of footage that you would rather not be on there, just delete that clip before anyone in authority can see it.