One thing which seems to be allowed, however, is for a politician to take a job with a company after having used the power entrusted to them by the electorate to benefit said company. Private Eye reports on this sort of thing all the time. They call it the Revolving Door. I can’t see any principled reason to think that undeclared future interests compromise the integrity of the politicians involved much less than undeclared present interests would.
It seems to me that it would be pretty easy to put a stop to it. You say that politicians giving contracts to companies have to declare an interest or either face trouble or not take a job with the company for, say, five years. And if people keep waiting out the time limit and then immediately taking jobs with companies they’ve helped, we smell a rat and extend the time limit. But perhaps I’m missing something and a rule like this would be impossible to put into practice.