This is a honest un bais opinion mate.
Breaking it - Ok, some parts will fly out, but you will be left with a 80% complete, un drivable car. That would then have to be trailered / craned to a scrappy for poo scrap value.
Selling as is - Fast easy money, although your price would be depending on the person buying, and the state of the car.
Sticking it in for an mot, even if it fails, the new buyer will konw what it failed for.
At the end of the day, without mot, it wont get you much money. So if you are that desperate for £300, sell it as it is now, and if your not, fix it and sell it, bag the profit. Or keep it.
Breaking is for someone with space and time to sit on a cheap investment (probably the new owner!)