Residual value
The other thing that's missing is that plant usually has a residual value even when it's depreciated to zero. Though Rodney trashed the gearbox after 4 years, a new gearbox is a few grand and even unrepaired, the lorry could be sold for £50k perhaps. That comes straight back to the company as income to be added to profits & therefore taxed. So Farnsworth's argument holds even less water.
When I stopped being a self-employed consultant, I had to look at the kit I had bought - server, UPS, laptop, etc - and written off in the first year under capital allowances. It still had residual value so I had to nominally pay myself the value of it and that went into the business accounts as income to go into the tax calculations.
Of course, as Tim says, the "£93Bn subsidy to business" is now a Lefty article of faith, and won't readily be dislodged by rational exposition from Tim or anyone else:(
Biting the hand that feeds IT
