It's entirely practical, but if they'd helped you in a significant way, they would not be allowed to lobby you. The other way to go is, if they had those ties and they were lobbying you, you would have to recuse yourself because of the favours they did before.
That threshold is already in the Lobbying Act and the Lobbyists' Code of Conduct. If you've helped someone significantly, you cannot lobby him or her for four years.