There's nothing to stop you from MOT'ing it on the 8th though. If the expiry date was the same as the test date then you could steal a day from them every year instead
IIRC, they aren't stealing a day, they're just forcing you to re-MoT the car a day before it expires. Next year's MoT will run from the 8th to the 7th in your example as well, it just that the actual test needs to be done by the 7th.