How's it going?
Of course, you're right: "Anyone that financially supports a certain political party obviously want their interests promoted." True enough. But I guess would emphasize 'anyone' as the operative word.
What is a 'corporation'? Is a corporation an 'anyone'? What do you mean by the term 'corporation' in the context of political funding? Are we talking about the ENTIRE corporation, manager, executives, and wage-labor employees down to the janitorial staff (which is probably contracted out, come to think of it)?
Or, are we talking about the board of directors of a 'corporation' and their biggest investors in the corporation?
Now, you juxtaposed 'corporations' (board of directors and their biggest investors) alongside "Labour Unions" [and] "other pressure groups."
As I understand it, Labour unions, for example, do go through some kind of democratic (small 'd') process as far as deciding where to put their collective funds politically -- and they do this among the ENTIRE membership of the union.
I would be surprised to hear that corporate boards (and their investors) take a poll among the entire 'membership' of the enterprise (executives, middle management, supervisor, hourly employees and janitorial staff) in order to determine where to spend the corporate funds (which everybody, the entire membership of the firm from the CEO to the toilet scrubber helped to generate) politically.
The board of directors (and their biggest shareholders) probably make those decisions among themselves with no input from down below. So, it really, really, really (Did I say 'really'?) depends on what, PRECISELY, one means by "corporations," "Labour Unions," and "other pressure groups."