I can't help but observe that your student informants appear only to be able to see one side of the coin, and an imaginary depiction of the coin at that. I hope, for your sake, that this very blinkered approach to situations was not a result of their education.

In this case, Tory donors tend to vary over the years with no direct payoffs. Most of them aren't much affected by corporation tax. A number were supportive of the Blair Labour Party. Trade Unions have a far more significant impact on the Labour party, being the deciding factor in the election of the last two leaders; they also directly sponsor a number of Labour MPs - and MPs have to compete for their support by espousing the union agenda.

All parties depend on donors to fund their campaigns. Membership fees etc aren't sufficient. Small donors haven't had much impact either. And the public is understandably antagonistic to the idea of having to fund politicians political campaigning out of taxation. So we are where we are.

You also, in passing, raise a more general point about corporation tax. There is a perception on the left that this is a large pot of potential money that is being left insufficiently taxed. Most governments, of all persuasions, would probably agree, but the problem is that corporation tax at high rates no longer works. Multinationals simply make all their profits abroad; Apple keep all their money abroad. Ireland has recovered from the financial crisis by attracting companies using a very low tax rate. There's a similar problem with the very rich.
And a not entirely unrelated issue with having higher rates of tax for higher earners.
You don't maximise the tax take by simply increasing tax rates, and it is only the tax take that is really important to governments. There is an optimum point; I'm not saying we are at it, but working out what it is is complex and subject to competition between countries.

xcity

I have, in my time, met many trade union leaders (in many countries).

Your post libels them.

The conclusions of some is -
Conservatives- paid by the few for the few
Labour -paid by the many (substantially through Unions) for the many

Corbyn has definitely got a battle on his hands when one considers his funding versus that of TM.

Others may conclude - a case of the blind leading the blind