* A friend asks me to borrow $400K. He is a good friend, and I am happy to help him out
   * A few months later, he decides to donate $200K to a political party that happens to be in power at the time
   * The government makes certain decisions that appear to be in my favour. I discussed that certain changes would be helpful but never asked for them directly.

Basically, it is a gentleman's agreement where no party involved explicitly asked for anything. Illegal in the spirit of the law, yes, provable in court beyond a reasonable doubt, no.

As another poster highlighted, you don't even need to use a friend in the UK, you can donate directly. The price of a seat in the House of Lords is £400,000 ($500K), but you can get other political favours much cheaper.